By the 12th century, these manuscripts as well as the commentaries on them, made their way back into Europe by way of Spain, Sicily and North Africa. And all of this was due to the Crusades and the reactivation of trade which the end of the Crusades made possible. By the middle of the 13th century, French and Italian universities were literally inundated with these ancient texts, especially the philosophical works of Aristotle. Aquinas studied Aristotle like no other man had before or since and he used Aristotle to justify his entire thinking. Rather, his theory of knowledge is a sober statement of how men know the world.
Man is a rational animal and the world can be understood by human reason.
A being endowed with reason, man can understand the universe. But as an animal, man can know only that which he can experience with his senses. This is Aristotelianism to the core. This brand of empiricism sets limits to what we can know. Whatever exists, can be understood.