Animalism and Brains
Alexander R. Pruss
February 5, 2007
Animalism is the view that we are animals and, thus, satisfy the criteria of identity proper to animals. This is highly plausible, for instance because it accepts at face value what appears to be the obvious facts that we are mammals—after all, we have the hair, the inner ear bones and the milk that mammals do—and that being a mammal is a way of being an animal. On the main opposing view, one has to hold that associated with each of us there are two entities: a person and an animal, of which we are only one, and this seems needlessly theoretically complex.
At the same time, many philosophers deny that we are animals, often because of the Brain Argument. The Brain Argument is:
The argument is valid, as long as we read the modality in (2) as de re. I shall make a two-part response. First, I shall argue that we all should accept (1). Second, I shall argue that it is consistent with animalism that one reject (2). In fact, I will show that it is quite plausible that human animals could survive losing all of the body except the brain. Finally I will consider objections to the idea of an animal’s surviving while preserving no part of its body outside of its brain.
In this paper, I will not consider the Upper Brain Argument, which is like the Brain Argument but with “upper brain” substituted for “brain”. I think (1) becomes significantly less plausible when that substitution is made, but a full defense of animalism would, however, require thorough consideration of the Upper Brain Argument. Nor will I consider similar arguments about survival as a soul which are relevant to some forms of animalism, such as that of Aquinas.
I will use two terms in a technical sense. “Rest of body” and similar expressions will refer to the human body except for the brain. A “prosthesis” is a mechanical device that performs some or all of the functions of a bodily part but is neither maintained nor nourished by the body in the way that paradigmatic bodily parts are. I will argue that prostheses are not parts of a body or an animal. However, if a mechanical device came to be appropriately maintained or nourished by the body, it would cease to be a prosthesis by this definition. Whether it would thereby become a part of the body is a difficult question that I shall not answer here.
For my argument I shall need the Prosthesis Thesis that no prosthesis is a part of an animal. Why should we accept the Prosthesis Thesis? I shall give three reasons.
i. Contact lenses. Surely contact lenses are not a part of the body. But sometimes they perform some of the functions of the lens in the eye. For instance, if the lenses in the eye are insufficiently convex, then the contact lenses will do some additional focusing of light, which additional focusing of light should have been done by the natural lenses in the eye. Thus, sometimes the contact lens is a prosthesis, and surely it is not a part of the human animal. Moreover, there does not appear to be a relevant difference between a contact lens and other prosthetic devices, and so we should accept that no prosthetic devices are parts of the human animal.
But perhaps there is a difference between prosthetic devices that perform some of the functions of a body part and ones that perform all of the functions of a body part? However, quite likely no prosthetic device performs all of the functions of a body part, and yet this limitation does not appear very significant in light of the fact a bodily part does not cease to be such by ceasing to perform some functions, and likewise a prosthetic part should not become a bodily part by gaining some more functions.
ii. Life-support machines and caregivers. Suppose we replace Jones’ heart by a roomful of medical machinery. That roomful of medical machinery is a prosthesis. But we have little temptation to say that it is a part of Jones’ body, so that Jones has grown to fill the room and her consent must be asked before replacing a part of the machinery since that replacement is a surgery. But the size of an organ surely does not matter. An artificial heart that is a prosthesis, i.e., an artificial heart that is not maintained or nourished by the body, is not a part of the body regardless of whether it fills the room or sits neatly inside the chest.
If someone is tempted to suppose that the roomful of medical machinery is a part of Jones, consider a twist. A motor breaks down in a piece of equipment, and Dr. Smith, who is quite an athlete, takes over for the motor, manually rotating its shaft at high speed. Then Dr. Smith is a part of the prosthesis. But does she then become a part of Jones?
And is a seeing-eye dog a part of a visually impaired human animal? Surely not. Of course, one might posit that nothing that is an animal in its own right can be a part of another animal, so that the seeing-eye dog could be held not to be a part of the visually impaired human animal, but a seeing-eye robotic dog would be. But even if one did this, the idea that a robotic seeing-eye dog is a part of the human animal is quite implausible.
iii. Ownership. If a human bodily part can be owned at all, it can only be owned by the person of whom it is a part. Bodies cannot be owned by another—that would in effect be slavery—and hence neither can their parts, since otherwise someone could just buy up all the parts of one’s body, and then the whole body would be owned.
But a prosthesis can be owned by someone other than the person in or on whom it is found. One can try on a prosthesis owned by someone else. One may transfer the title to a prosthesis, while keeping the prosthesis in or on one’s body, e.g., with the understanding that the use of the prosthesis will transfer to its owner upon the user’s death. One can lease a prosthesis. And if a one-legged man facing bankruptcy were to liquidate all his assents, turn them into diamonds and gold, and then have an artificial leg fashioned out of the gold and diamonds, it would be reasonable for the court to order the artificial leg sold, and a (more practical!) wooden or fiberglass one to be acquired instead, although no bankruptcy court would have the right to order a body part sold.
Objection. “Many prosthesis users treat a prosthesis as if it were a part of themselves. Thus, just as it is generally inappropriate in American culture deliberately to touch a stranger’s body without consent, it is inappropriate under the same circumstances to touch a stranger’s prosthesis without consent.”
However, note that this treatment of the prosthesis as a part of the self can include one of the cases where it is clear that the prosthesis is not a part of the body—the seeing-eye dog. Thus the intuition here is one we need to be cautious of. Moreover, our rules of etiquette likewise have prohibitions against touching the clothes, cars and other property of strangers without their permission.
Now, given the Prosthesis Thesis, it is time to move to grisly thought experiments that establish that we could lose the rest of the body and yet survive.
Let me begin with a somewhat less controversial fact. I could survive the loss of everything from the neck down. Imagine that life-support machinery is set up to pump blood in and out of my head, and the veins and arteries of my head are disconnected from the body below the neck. It seems clear that I could survive this. In fact, I could watch this procedure, rapt in horror (to keep me conscious it might be necessary to directly anesthetize the pain centers in my brain, and whether I could be kept sane is an open question).
Suppose, next, that my spinal cord is severed in my neck. I would then experience the loss of all sensations from the body below the neck, and I would lose all control over that portion of the body. But I would continue to receive sensations from my eyes and ears, and so I could be conscious throughout the procedure. There is very little plausibility to supposing that as a matter of logical necessity I would perish.
At this point the body below the neck is paralyzed, and does not supply the brain with oxygen or nutrition. There is nothing impossible about watching it get cut off. This would be a grisly procedure, but one could surely survive it.
We could imagine a science-fictional surgical procedure, in fact, where the head is disconnected from the body, and then surgery is done on the body independently of the head, which is then reattached to the body. There is nothing absurd about the idea that one could still be conscious, and hence still be, while the body below the neck was detached from one’s head, and one wouldn’t be where that torso and limbs were, because it is with the brain that the mind is seems most clearly associated. Thus it is possible to survive with no bodily parts beyond the head.
I could, thus, survive with my body having no parts outside the head. Suppose then that I have no bodily parts outside my head. Next, suppose I lose the ability to receive any tactile sensations, whether of pressure or temperature. This does not seem to threaten the logical possibility of my conscious survival. I could also lose taste and smell with no loss of consciousness. This would leave me with two senses: hearing and sight.
Now I think I could survive, though I would soon become insane, if I lost hearing and sight, too. But I do not need to argue this. For I could plainly survive the replacement of my ears with microphones hooking directly into my brain and of my eyes with cameras also hooked into my brain.
At this point, what would be left of my body would be a brain, a skull, some facial muscle, skin, and a few organs of relatively little use at this point, such as a tongue that neither tastes nor speaks. What is left outside of the brain is of much less significance than what was already lost. It seems highly plausible that I could lose the remaining organs, one by one, leaving only the brain and a mass of prostheses. It would be logically possible to be conscious throughout the procedure.
If this is right, then I could survive with the brain and its parts as my only body parts. Note that here I need to make use of the Prosthesis Thesis to block the response that the cameras, microphones, pumps and other equipment that replaced the rest of the body would in fact have to be parts of the body. If one could make such a response, then the Brain Argument would be significantly less plausible, since it would be much harder to give a scenario on which one survives with no bodily parts outside the brain. But the Prosthesis Thesis implies we do not need to worry about this.
Consider a second line of reasoning. What matters most to our conscious survival besides the mind and brain are vital systems such as the circulatory system, the lungs, the digestive system, as well as our sensory organs. But we have little temptation to deny that we could survive the loss of our muscles, of our reproductive system, and of our tactile and temperature sensors, as well as the prosthetic replacement of the heart, blood, lungs, kidneys, liver, digestive system and sensory organs. In such a case, granted, there would be more than just a brain there. There would be a brain, bones, skin, and several other organs. But it does not appear plausible that the bones, skin and remaining organs are crucial to our continued conscious survival. Hence, we could survive pared down to the brain.
A related line of argument is to note that there is no part outside the brain that we could not consciously survive the prosthetic replacement of. But supposing that such prosthetic replacement gradually continued part by part, would we then fade into unconsciousness and non-existence? There may be some plausibility to thinking this would be true if the replacement continued into the brain. But if the replacement stopped outside the brain, it is hard to suppose that we would at some point perish.
We all thus have good reason to accept that it is logically possible to survive the loss of all one’s body parts outside of the brain. Notice how I have formulated this. I did not say that it is possible to survive just as the brain. For if I did that, the dualist would disagree with me. Nor did I say that it is possible to survive with one’s body being identical with one’s brain. For those who accept a co-locationist solution to the problem of material constitution might well insist that in my reduced state there would be two entities, I and the brain, that would be co-located.
Further, observe that nothing in what I have argued shows that I am just a brain. We all agree that I could survive loss of my legs, but it is also clear that if I am lucky enough to have legs, then they are a part of me, albeit not an essential part.
I shall now give several arguments that some higher animals, in particular human ones, can survive without the rest of their bodies. I will then consider several arguments against such survival.
An animal can survive the prosthetic replacement of some of its parts, as long as “not too much” is replaced. But how do we measure the amount of replacement? The simplest measure is size and weight. This, however, would lead to absurd conclusions. We could imagine an animal suffering from something like elephantiasis where the affected limbs were quite a sizeable proportion of the animal’s body size and weight, but the size and weight of the limbs would be completely irrelevant to the question whether the animal could survive their prosthetic replacement.
Elephantiasis can be handled by supposing that what determines the possibility of survival after prosthetic replacement of a collection of parts is not the actual size and/or weight of the organs, but the proportion of the size and/or weight of the organs to the size and/or weight of the organism in a normal individual. But even this is implausible. There is nothing absurd about a species of mammals evolving whose normal limbs were quite a significant proportion of their normal size and weight, but even there we would not say that the mammal is unable to survive the replacement of these limbs with prostheses.
Nor will counting the number of organs do, so that an animal counts as perishing when too great a percentage of the number of organs is replaced by prostheses. For it seems reasonable to say that each cell is a kind of functional organ, and then the hypothetical mammal with very big legs containing very many cells will still be a counterexample.
If it is not the organ count, size or weight that matter, perhaps it is the importance to survival that we should consider. Now, except possibly for the brain, every organ essential to the life of a mammal is such that the animal could survive that organ’s being prosthetically replaced, as long as that organ were the only thing replaced. Fido would be the same dog after getting an artificial heart. But what we need to figure out is whether an animal could survive the collective prosthetic replacement, simultaneous or sequential, of all of the parts other than the brain. And for the importance-to-survival measure to be important, we would need a way of comparing the survival importance of collections of organs.
But it seems unlikely that such comparisons make sense. Of the heart, the brain and the lungs, which is the most important to the survival of a mammal? In the course of nature, if a mammal loses one of the three, it dies. It dies, let us say, slightly faster when it loses the heart than when it loses the lungs. Do we, then, measure the survival importance of an organ or collection of organs by how fast the organism would die without the organ or the collection? Let us try this option. But how would we make this counterfactual precise? A human might bleed to death upon losing a hand. Is that to be taken into account? Or are we to imagine that the organs are removed in a science-fictional way with no damage to the remaining portions?
A more serious problem is that to apply this definition we need to have a criterion of death, and yet we want to use this definition as a criterion for what kinds of replacements can be survived. Suppose that it turned out that an adult mammal logically cannot survive without a liver—then the time of death would be immediate upon the destruction of the liver, even if everything else apparently continued functioning.
Moreover, even if we can fix up all the problems, say by talking not of the time of death but the time of the cessation of the organic functions of all the body parts, the time of death criterion gives the wrong answer. An animal would die about as quickly as imaginable if all of its blood vessels disappeared. There are few faster ways for it to die, except perhaps by instant incineration and the like. Yet it seems highly plausible that an animal could survive the replacement of all of its blood vessels with prosthetic tubes.
Perhaps instead of looking at the importance of a collection of organs to the functioning of the organism, we should look at the contribution of a collection of organs to the distinctive functioning of the organism as distinguished from other kinds of organisms.
If we do this, then in the case of human animals the brain may begin to loom large. Of all the bodily organs, it appears that it is the brain that is most explanatory of the differences between the functioning of human animals and similar non-human animals. Our opposable thumbs are nice to have, and their presence may have driven the evolution of the brain, but being as we are, our functioning would not change that much if we lost our opposable thumbs, whereas it would if our brains were changed.
Congenial to my position that we could survive the loss of the rest of the body as this is, this criterion probably cannot stand. For which other kinds of organisms should we be comparing the animal in question to? Should we compare it to all other organisms? If so, then one might argue that our circulatory system may be just as explanatory of the differences in functioning between human animals and E. coli bacteria as are our brains. But human animals could probably survive the replacement of the circulatory system with a prosthesis. And how do we quantify the contribution of collections of parts. How much more of a difference between E. coli and humans is accounted for by the heart and kidneys than just by the heart alone? And how do we weight the contributions to differences in functioning vis-à-vis different comparison species when we quantify the overall contribution to the difference in functioning that a collection of organs makes? It seems hard to imagine non-ad hoc answers to these. So the comparison to all other organisms does not seem appropriate.
But perhaps the relevant measure is the contribution of an organ to the differences between the functioning of an organism and the functioning of organisms in the species closest to it. However, this would give the wrong answer in hypothetical cases. Suppose that a new dog-like species evolved out of dogs. The members of the new species are just like dogs, except that the males have bumps on their skulls with which they butt sexual rivals. It seems exceedingly implausible to suppose that an animal from that species would count as perishing upon the removal of the bumps, even though the bumps are what would account for the difference in functioning with the closest species, the dog.
But perhaps instead of counting organs, we should count the DNA base-pairs that code for a given collection of organs. The problem with this is the possibility of inefficient DNA coding. Suppose that we found out that the majority of our DNA is used to code for our four limbs, and that it did so in an extremely inefficient way. We ought not conclude from this that the human animal perishes when its limbs are replaced with prostheses.
One might instead try to count the hypothetical number of base-pairs in an optimal DNA coding for a functionally equivalent organ. But why should the count of base-pairs in a hypothetical DNA coding be relevant? I think the only reason why such a count is relevant is because it reflects the functional complexity of an organ. Let us then consider functional complexity.
Functional complexity is itself hard to quantify, though we do have the intuition that fingers are more functionally complex than our earlobes, and our eyes than our skin. And even without a definition, we do have a rough-and-ready way of checking how functionally complex an organ, namely by seeing how complex a prosthesis for the organ’s central functions would have to be. On the side of the prosthesis, the complexity can be intuitively estimated in terms of the number of parts, the sensitivity of the parts to the particularities of its placement, and so on.
The very fact that we can already make a replacement for a heart but are very far from making one for a brain is, then, evidence for a greater functional complexity in the brain, as is the fact that we understand the functioning of the heart much better than we do that of the brain, even though enormous amounts of research funding have been poured into the study of each. Indeed, it seems quite plausible that a prosthesis for the brain sufficient to perform the brain’s functions would be much more complex than a single prosthesis for all the rest of the body. This is particularly likely if we did not have to mimic the details of the internal functioning of the parts of the rest of the body, but only the external functioning and its interaction with the brain, so that instead of building an immune system, we could just make the body out of metals and ceramics impervious to viruses and bacteria. And a functional account of complexity should in fact allow this.
If this is correct, then it is plausible that a human animal might survive the prosthetic replacement of the rest of one’s body but not of the brain. We may have some intuitions to the contrary, but at least some of these can be explained by a size-bias where we think of larger parts as more important and an appearance-bias where we think of visible parts as more important.
A crucial philosophical difficulty in talking of replacing an organ with a prosthesis that fulfills the same function is with the concept of function. Attempts to define proper function in evolutionary ways met with only limited success(??ref). That said, we could just stipulate that what we mean by “function” here is what these evolutionary accounts say, and bite the bullet on difficult cases. Alternately we might note that the concept of the function of an organ is arguably required for making conceptual sense of what the task of a physician is(??refs), so the concept of function is not brought in ad hoc—we need this concept, and have at least intuitive grounds for judging of the function of an entity. Besides, the larger argument here is that animalists for reasons internal to animalism could or perhaps should conclude that we can survive the prosthetic replacement of the rest of the body. But many animalists are Aristotelians, and as such already committed to a teleological metaphysics that includes the concept of function.
Thus, what may be the most plausible way of rendering the “not too much” requirement on replacement leads us to the idea that human animals might well be able to survive the replacement of the rest of their bodies, leaving only the brain intact.
What was said here is not some ad hoc way of transferring intuitions about personal identity into intuitions about animal identity. For it has implications for other animals whose brains have significantly more functional complexity than the rest of their bodies—other primates are surely like that, for instance, and it may even be the case that all mammals are like that.
The problem here is not just a problem for animalists. Everybody who believes that there are such things as animals and that they can survive some but not all prosthetic replacements faces this problem. Given the plausible intuition that what matters is “how much” has been replaced, we need a measure of “how much”. Gross size and weight is a non-starter. Counts of organs or DNA base-pairs are likewise implausible. Rather, some sort of functional criteria are needed. I examined three: contribution to survival, contribution to distinctive functioning, and functional complexity. Contribution to survival did not favor the importance of the brain, was untenable. Contribution to distinctive functioning favored the importance of the brain on one construal, but was untenable on any construal. But functional complexity favors the brain, in humans and other primates.
Suppose an organism has two parts such that part A controls part B, but not vice versa. Then, I claim, the destruction or prosthetic replacement of A is more likely to contribute to the destruction of the organism than the destruction or prosthetic replacement of B.
For if A is a controlling part, then insofar as B is controlled, it actively functions as a part of the organism precisely when it engages in its characteristic activity under the direction of A. Thus without A, part B insofar as it was controlled ceases to actively function as a part of the organism, but A can function as a part of the organism without B. Thus, A can send out controlling signals or movements in the direction of where B was, and these signals or movements are part of the active functioning of A. But B, insofar as it was controlled, is no longer actively functioning as a part of the organism without A; it can only await control. Now if A was prosthetically replaced, then B can continue to contribute to the welfare of the organism, but it now lacks the organic interconnection with the rest of the organism that made it actively function as a part of the organism.
In fact, in this case, B is itself almost like a prosthesis. For imagine that someone had a leg severed above the knee, and then doctors built a mechanical leg that had electrical motors instead of muscles. But, upon the patient’s request, they built the mechanical leg around the bones of the severed leg. The movement of these bones is now controlled by electrical motors instead of muscles. It seems that in this case the bones are hardly a part of the body—we just have a prosthesis that happens to be in part built out of organic matter derived from the original.
In fact, it is plausible that if a part of the body controls and coordinates the functioning of several other parts, then, at least typically, the organism can survive the prosthetic replacement of the controlled parts at least as long as the controlling part continues to control and coordinate the prostheses, but perhaps not the other way around. For a vivid image of the asymmetry, imagine replacing the arms of the octopus by mechnical prostheses versus replacing the central portion. If we replace the central portion, then literally the arms are no longer organically connected to each other—they are mechanically connected to each other. But if we replace the arms, then it seems that we still have the same octopus.
The intuition about the replacement of controlled parts can be argued for on the following grounds. At least a part of what defines an organism as a living organism of its natural kind is not the actual performance of life functions but something more like a striving for such performance. Think of the fish out of water which struggles to extract oxygen from its unnatural environment. As long as it is striving, it is alive.
Now if the controlling part is no longer there, then it is difficult to attribute to the organism any of the strivings of the controlled parts. If a heart is stimulated so that it keeps on beating after being disconnected from an animal, its beating is no longer a part of the organism’s striving for oxygenating the internal organs. However, if there is no longer a functional organic heart but the lower brain continues to send signals that control the heart rate, then it makes sense to say that the organism continues to strive to oxygenate the internal organs, even if this striving will be unsuccessful. Thus if the controlled parts are replaced by prostheses but the controlling part remains functional, it appears possible to attribute the striving for performance of life functions to the organism.
Now, at least in vertebrates, we can say that there is a fairly well defined nervous system that controls and coordinates the functioning of the rest of the organism. Therefore, prosthestic replacement of everything but the nervous system should preserve the identity of the organism. It follows that a human animal could survive the prosthetic replacement of everything outside of its nervous system.
Moreover, it is not necessary that all of the nervous system survive. For instance, the parts of the nervous system that merely carry signals can be thought of as under the control of other parts, and hence are not necessary for survival. But likewise neither are all of the controlling portions necessary for survival, but only a sufficiently substantial part of them are needed, a part large enough to identify the strivings of the remainder as strivings of the same organism. It is very plausible that the brain would be such a substantial part without the spinal cord, even thought the spinal cord has some autonomous control.
Of course there is a difficulty here in specifying what counts as a “sufficiently substantial part”. We would not say that a mammal survived if everything but the spinal cord were destroyed. Perhaps the specification should be done in terms of functional complexity as in the previous section. Or perhaps one might note that while the brain exercises some control over the functioning of the spinal cord, the spinal cord does not seem to exercise similar control over the functioning of the brain.
Therefore, in those animals where the brain forms a sufficiently substantial part of the nervous system and the nervous system is sufficiently controlling, the animal can survive the destruction of everything but the brain, assuming prosthetic help. This is true of the adult human but may not be true of reptiles.
Note that on the control criterion, whether a human animal can survive prosthetic replacement of everything but the brain can depend on the stage of development. Thus, in a human embryo at a stage where the brain has little controlling functionality, the animal might be able to survive the destruction of the brain, and will not be able to survive the replacement of everything but the brain. This is how it should be. We need to be open to the possibility that as an animal develops, the answer to the question of which parts are more central to its organic functioning may change.
Objection. “There is no way to distinguish control from input. But the brain receives inputs from the rest of the body. Thus it is just as much controlled as controlling, since it is just as much inputing as outputing.”
Distinguishing control from input is indeed difficult. In the case of voluntary functions, however, it does appear to make sense of the idea that the senses, say, provide information and the animal then decided on what to do in the light of this information, and there is certainly an intuitive difference between providing input and controlling the function. The brain appears to have a certain autonomy vis-à-vis its functioning that the muscles do not. Perhaps this autonomy needs to be spelled out in terms of indeterminism or maybe in terms of functional complexity.
The distinction between a diffuse and a non-diffuse cause may also be helpful here. Thus while the brain’s outputs are affected by the inputs, the inputs are at most a diffuse cause of the brain’s producing such-and-such outputs. They are diffuse in the sense that inputs from multiple sensory organs are, typically, involved in producing an output from the brain—the brain needs to integrate these inputs—whereas the brain is a less diffuse cause of the movement of the muscles, say. (It won’t do to imagine a case where there is only one sensory organ left in a human animal; for then the absence of data from other organs is still an input, though a negative one, that the brain receives.)
Finally, let me consider a distinctly Aristotelian approach. We are animals, but we are rational animals. What defines a living organism is its teleological structure, and especially its distinctive ends. It is when we function best qua rational animals that we flourish simpliciter. There is such a thing as the distinctive proper functions of an animal, and in the case of a human this is agency. There is a teleological relationship between our sub-rational faculties and our rational ones, in that the sub-rational faculties exist for the sake of the development and/or sustenance of the rational ones. Thus our heart and foot exist for the sake of our rationality. Biologists may not see this, but biology should have no monopoly on the study of the human animal.
The teleology here can be understood in one of three popular ways. First, and probably least satisfactorily, one might take an evolutionary view of it, and say that as the evolutionary success of the human species came primarily from intelligent agency, we can therefore think of the development and/or sustenance of intelligent agency as a central aim of all our body parts. Second, one might take a theistic view, defining teleology in terms of divine plans. Thus, God gave us heart and foot to sustain our agency. Third, one might take organic teleology to be a basic feature in the world, not reducible to historical facts such as those about selective advantage or divine agency.
But it is not my purpose here to defend this teleological understanding of the human person. I can simply restrict this argument to those who accept the teleological presuppositions. Since my main purpose in this paper is to show that animalists have reason internal to their animalism to accept the possibility of surviving the destruction of the rest of the body, all I need to do here is to address myself to those animalists who are Aristotelians and who, likely, were left unsatisfied by the non-teleological nature of the arguments in the previous two sections.
On a teleological view, it makes sense to suppose that an organism can survive the loss or prosthetic replacement of what more mediately supports and/or promotes the distinctive proper functions of the organism when that which more immediately supports these functions is maintained. What we get is now structurally like the control view, except that immediacy of the connection to the distinctive functions takes the place of control. Thus, the heart and lungs only support the agentive functioning of the human organism insofar as they support the functioning of the brain and muscles.
Moreover, it is plausible that an organism can survive as long as some portion of its distinctive functioning remains. But if one replaced everything but the brain with a prosthesis, it seems highly likely that agentive thought could remain, and could exercise some agentive control, if only over the brain’s outputs.
I am not claiming here that the continuation of distinctive teleological functioning is necessary for survival. Human animals while dreamlessly asleep do not, as far as we know, engage in their distinctive proper function of agency. But that is not a problem. For the human animal is not to be identified with its proper function, but with the subject of the proper function. And that subject remains in existence while the human is dreamlessly asleep. What continues to exist during deep sleep is the capability for such functioning, and that capability’s subject, namely the animal.
For a similar reason, a human animal can count as already having existed as an embryo, since there is a striving in the direction of the later exercise of proper function, a striving exhibited through the cell specialization as guided by a genetic code that codes for the brain. On the other hand, if we completely destroy the brain of an adult human animal but artificially sustain the functioning of the rest of the body, then neither is there a capability in that body, remote or close, for such distinctive functioning, nor can we say that what remains is striving towards such functioning. The case of destruction of a portion of the brain is more difficult to evaluate, but this evaluation is not necessary to achieve the purposes of this paper.
i. Identity considerations. “If I can survive as just a brain, then I already am just a brain. For suppose that tomorrow I will survive as just a brain. Call that brain x. Now this brain already exists today. Now, if I am identical with x tomorrow, and x tomorrow is identical with x today, then I am identical with x today. Hence, today I am just a brain. But this the animalist denies. Hence the animalist must say that I cannot survive as just a brain.”
To see that this argument must be resisted by an animalist, note that it is clear that a human animal could survive with its body reduced to a head, neck and torso, and the typical contents of these. But replace “brain” in the above argument by “combination of head, neck and torso”, and notice that the argument works just as well, or just as badly.
In fact, a version of the above argument is a standard opening gambit in discussions of the identity of material objects. Suppose Fluffy is a normal cat that tomorrow will lose its tail. Let “Fluffy-Minus” be the tailless cat. Then, if Fluffy survives the loss of its tail, tomorrow’s Fluffy will be identical with Fluffy-Minus. But today’s Fluffy is identical with tomorrow’s Fluffy, and today’s Fluffy-Minus is identical with tomorrow’s Fluffy-Minus. Hence, by transitivity, today’s Fluffy is identical with today’s Fluffy-Minus, i.e., Fluffy already lacks a tail, which we assumed was not yet the case.
There are many proposed solutions to this apparent paradox of material constitution, and most of these solutions can also help the animalist maintain that I could survive with my body reduced to a brain. For consider the following brief catalog of four of the more prominent solutions.
(1) Co-location: Fluffy and Fluffy-Minus are always distinct, but tomorrow they will occupy exactly the same area in space. But likewise the animalist can say that if tomorrow I will lose the rest of body, then tomorrow the brain and I will be two distinct entities occupying the same area in space, while today the brain and I are two distinct entities occupying different areas in space (the brain occupying a proper subset of the area I occupy).
(2) Denial of parts: Either entities have no parts (Toner??ref) or have only simple particles as parts (van Inwagen??ref), and so there is no such entity as Fluffy-Minus now, since there is only Fluffy, and possibly its particles, existing today with a tail and tomorrow without. The animalist can likewise say that there is no such entity is my brain now, since there is only I, and possibly my particles, existing today with a brain, heart, lungs, legs, arms, etc. (and possibly a soul), and tomorrow with just a brain (and possibly a soul).
(3) Relative identity: Fluffy today is the same cat as Fluffy tomorrow, but not the same lump of flesh and bone, since the tail is not there tomorrow. Likewise, I can say that I am the same animal as the brain tomorrow, but I am not the same lump of flesh and bone, being a much larger lump today.
(4) Denial of transitivity: Today’s Fluffy is the same as tomorrow’s Fluffy, and tomorrow’s Fluffy is the same as tomorrow’s Fluffy-Minus, but it does not follow that today’s Fluffy is the same as tomorrow’s Fluffy-Minus. In the same vein, I can say that today’s I is the same as tomorrow’s I, and tomorrow’s I is the same as tomorrow’s brain, but it does not follow that today’s I is the same as tomorrow’s brain.
The one solution that cannot help the animalist is anti-realism about wholes: neither Fluffy nor Fluffy-Minus exist, but only their indivisible parts do. If anti-realism about wholes holds, then we are either indivisible—and the only at all plausible proposal that makes that true is Cartesianism—or else we do not exist. The animalist rejects both options, as do many anti-animalists. Each of the other four answers has advantages and disadvantages. However, since the problem of material constitution is a problem that everyone except the person who does not believe she exists and the Cartesian opponent of wholes needs to face, the animalist does no worse than everyone except those who are insane or Cartesian or both.
ii. The brain is only an organ. “An animal is more than an organ. Hence, an animal could not exist as just an organ.”
This may simply be a variant of the previous identity argument. If so, I have already answered it. Or, more interestingly, it may be a worry that it is somehow essential to an animal to have a body that is more than just one organ. But here we may ask: Why? Take a microscopic animal which consists of a cell, together with external cilia. It makes sense to talk of the cell as a single, complex organ. Yet the animal could, surely, survive the loss of the cilia, and hence survive the loss of all its body besides one organ.
But perhaps one thinks that to have an organism one needs a multiplicity of organs working together. This, however, will not help the objector since the brain does have parts that are themselves organs, parts such as a hemisphere or maybe even a neuron.
Finally, the person making this objection may be a hylomorphic dualist who thinks that an organ by itself cannot be an animal, since only something with a substantial form can be, and the substantial form unites all the organs. However, this objection has no weight, for there is nothing absurd about the substantial form informing simply one organ, and uniting that organ’s interior parts.
iii. An embryo need not have a brain. “Plainly, an embryo is the same animal as the adult human animal. Otherwise, we would have the unanswerable question of when the first animal perished. But an early embryo has no brain. Hence it could not survive the loss of everything but the brain, since that would be the loss of all of its body.”
A hylomorphic animalist may object to the claim that the embryo could not survive the loss of everything but the brain by saying that embryos have souls that would survive such a destruction. But it is not necessary to insist on this in order to answer the objection. For the claim about embryos is irrelevant to the purpose of this paper which is to respond to the Brain Argument. The Brain Argument holds that we adults can survive the loss of the rest of our bodies besides the brain. Whether embryos can or cannot is irrelevant here. For the record, even when I have not explicitly stated this, my claims in this paper that one could survive with one’s body reduced to a brain were argued for only in the case of human animals with fully developed brains.
iv. Souls. “Dualism holds and human beings have souls enabling them to survive death. But an animal cannot survive death. Hence, animalism is false.”
This is a dualist version of the Brain Argument, with the soul in the place of the brain. As such, there is no need to answer this argument here, since it is independent of the Brain Argument. But it is worth noting that the control-based and Aristotelian arguments might allow an animal that has a soul as a part of it to survive as just the soul.
v. An animal could survive a brain transplant. “It is plain that if an animal had the brain of another animal transplanted in it, and its own brain destroyed, the body with its new brain would continue to be the original animal. Not so on the arguments of this paper, at least in the case of human animals, since on these arguments this would have to be seen as the donor surviving in the body of another.”
Here, I dispute the claim that the body with the new brain would be the original animal. I suspect that there is a size bias at work behind intuitions that favor the judgment in the objection. Suppose that we were dealing with an alien species whose brains were very large, and all of whose other organs were a thin layer spread over the outside of the brain. Then it would seem much less likely to us that the original animal would survive the destruction of the brain and a new brain’s being put in. Rather, we would describe it as the donor’s brain’s getting a new outside layer.
The intuition here is that the animal whose contribution to the final animal is greater is the one we hold to survive. But as we learned in Section 3.1, physical size is not how we measure the size of this contribution. And if we measure this contribution in terms of functional complexity, or consider the control or teleological aspects of the contribution, we are led to think that it is the animal whose brain survives that survives, getting a rest-of-body transplant.
vi. Homeostasis. “Animals are organisms. Organisms maintain homeostasis. Brains cannot maintain homeostasis. Hence animals cannot be brains.”
That this objection fails is clear when we consider the case of a prosthetic heart. An animal can survive the replacement of its heart with a prosthesis. But the animal as such no longer maintains homeostasis—it is only the animal plus the prosthetic heart that do so. It might be argued that this is only so if the prosthetic heart is under its control, in which case we could survive as brains in vats only if the vats were controlled by the brains. But that is incorrect—it surely does not matter to facts about the survival of the animal whether, say, the pumping rate of the artificial heart is controlled by the animal or not.
If, on the other hand, one allows an organism to maintain homeostasis by being a part of a larger homeostatic system, say the animal plus artificial heart, then we can likewise say that the brain in a vat is part of a larger homeostatic system, namely the brain plus the vat. Now it might be argued that the animal that had its heart replaced by a prosthetic continues to contribute to homeostasis—the heart is only doing a part of the work. But the brain makes some kind of a contribution, too. It is composed of cells, and each of these cells is a homeostatic unit, and these units are functioning together.
But perhaps an animal can only survive if it substantially promotes homeostasis in an organic way. This, however, would seem to get wrong the case of an animal on life-support, an animal whose poorly functioning body is, if anything, right now harming the homeostasis of the organism as a whole.
There may, however, be a kernel of truth to the homeostatic account. It may be necessary for something to live as an organism that it strive for homeostasis. But the lower brain strives to promote homeostasis by striving to control bodily organs. That these organs no longer exist to be controlled in the case of a brain in a vat does not remove the striving.
I have argued that on three plausible accounts, a human animal can survive with nothing but the brain. The Brain Argument is, thus, unsound. Whole-brain-transplant and whole-brain-death considerations, thus, provide us with no evidence against the thesis that we are animals.
 An animalist can also say that both an animal and a person exist and that they are distinct entities, as long as she also says that of these it is the animal that we are and not the person. But this view is very implausible.