"The true science and study of man is man."
In the 5th century B.C., the Greek sophist Protagoras wrote,
"Man is the measure of all things."
This statement is generally taken to refer to subjective truth, such as whether a woman is beautiful or a circumstance is happy. But it can just as well be said of objective knowledge, such as whether the boiling point of water is 99.97 degrees Celcius. Water may always boil under certain conditions whether man existed or not. But the ascertainment of that fact is dependent on the mental and sensory powers of man.
As David Hume wrote,
"'Tis evident, that all the sciences have a relation, greater or less, to human nature: and that however wide any of them may seem to run from it, they still return back by one passage or another. Even. Mathematics, Natural Philosophy, and Natural Religion, are in some measure dependent on the science of MAN; since the lie under the cognizance of men, and are judged of by their powers and faculties."
"There is no question of importance, whose decision is not compriz'd in the science of man; and there is none, which can be decided with any certainty, before we become acquainted with that science."
To be confident in our knowledge of things that are not self-evident, we must be confident in our method of learning about such things. To be confident in that method, we should discover first any self-evident truths about how we, as men, learn. There are no self-evident truths about other men. The only self-evident truth a man has access to lies within himself. As the science of man is thus most appropriately prior to all other sciences, the science of one's self is most appropriately prior to the science of man in general.
Or as Thales said,