Final answer:
Aristotle considered the First Cause or Prime Mover to be the firmest principle in metaphysics. This concept helps explain the nature of individual things and the universe as a whole.
Step-by-step explanation:
Aristotle considered the firmest principle of all in metaphysics to be the First Cause, also known as the Prime Mover. He believed that everything in the universe is composed of four causes: material cause, formal cause, efficient cause, and final cause.
These causes are interconnected and lead to the existence of the Prime Mover, which Aristotle considered to be 'thought thinking itself.' He believed that the Prime Mover is responsible for the motion and order in the universe. This concept helps explain the nature of individual things as well as the nature of the universe as a whole.