|Aquinas (painting by Carlo Crivelli)|
First argument: Every imperfect thing desires perfection. The desire for perfection is a thing’s desire for the actualization of its naturally essential potentialities. For a human being, this means achievement of intellect because it is through intellect that a human being becomes a human being. But the soul of a man is a blank slate on which nothing is depicted. Human beings do not possess any innate knowledge of reality. Only knowledge can lead to the actualization of natural human potentialities and therefore human beings have a natural desire to know.
Second argument: It is the natural desire of everything to function in a proper way—for instance, all objects released from a height will move towards a lower level. The function proper to man is the desire to understand—this is what distinguishes man from everything else that exists. Therefore human beings have a natural desire to know.
Third argument: It is the natural desire for everything to be united with its principle source because that is the way by which a thing can attain perfection. Aquinas gives the example of circular motion, which is perfect because in it the terminus is united to the beginning. A human being can be united with its source only through the means of intellect. Therefore human beings have a natural desire to know.