Final answer:
Aristotle argues for a principle of pure actuality to avoid an infinite regress of causes, positing a First Cause or Unmoved Mover, which Aquinas aligns with the concept of God.
Step-by-step explanation:
Aristotle contends that there must be a principle whose essence is actuality because the existence of the universe needs an explanation that cannot come from an infinite regression of causes. Instead, there must be a First Cause or a First Mover that itself is not moved or caused by anything else. This is necessary to stop the infinite regress and provide a starting point for all motion and change in the universe.
The Unmoved Mover, for Aristotle, is purely actual without any potentiality, meaning it is entirely self-actualized and does not change or move. It's the ultimate cause of all motion without itself being moved. This concept inspired Aquinas's cosmological argument, where he connects the Aristotelian Unmoved Mover with the Christian conception of God, a necessary being who causes the existence of all contingent beings and imparts purpose to them.