Five Ways, Part I

The First Way, as explained by the Summa Contra Gentiles

• Why believe that nothing moves itself?
  • Argument I
    • Reduce to case where the whole is moving (transitive) itself, and the movement is "primary".
    • If one part were at rest, the whole would be at rest.
    • But if the resting of the part would cause the resting of the whole, the movement of the part must be causing the movement of the whole, and hence it is not a case of self-movement.
  • Argument II
    • Exhaustive list of kinds of change: accidental, by force, by nature.
  • Argument III
    • Nothing is in act and in potency in respect of the same thing.
    • But the thing moved is in potency--it is having its potency for motion fulfilled.
    • The mover is in act.
  • A different way.
    • Suppose that an entity started itself moving at t.
    • The transition from not causing itself to move to causing itself to move is a kind of change.
    • Could the entity have initiated that transition? But then a regress follows, and we still don't have the start of self-movement.
    • What if the entity was always moving itself? Then we don't have the transition, but we still have a regress in the order of explanation: Why is the entity causing itself to move?
• Why no regress?
  • Argument I
    • Moving is simultaneous with being moved.
    • But can't have an actual simultaneous infinity of movements. For an infinite body cannot be moved in finite time?
  • Argument II
    • If one takes away the first mover, the whole series disappears.
    • So we cannot have a series of merely intermediate movers.
    • But if all we have is an infinite series (or, better, ungrounded infinite series), then that's all.
  • Argument III
    • Instrumental movers require a primary mover.
• Variant argument in SCG.
  • It is either accidental that each mover is moved or not accidental.
  • If it is accidental, then it could be that no mover is moved. Thus, it could be that there is no motion, and hence it could be that there is no time.
  • Moreover, if it is accidental, it is likely that somewhere this fails, and hence that there probably is an unmoved mover.
  • If non-accidental, then each thing is moved by the same kind of movement that it has, since there are only finitely many kinds of movement. But that is not right.