1. Based on motion. Aquinas included not only movement from one place to another, but also movement in the sense of change of quality or quantity
2. An object only moved when an external force was applied to it
3. This chain of movements or changes cannot go back to infinity
4. There must have been a first mover, which itself was unmoved
5. The 'unmoved mover' began the movement in everything, without actually being moved
6. Fire when applied to wood changes the wood, and gives it the potential to become hot
7. In order for a thing to change, it requires actuality. If it did not, a thing would have to initiate change in itself, which would require that it is both actual and potential at the same time
8. Aquinas considered this to be a contradiction