Discourses: Book 2 17 : 1

What is the first business of one who practises philosophy? To get rid of thinking that one knows; for it is impossible to get a man to begin to learn that which he thinks he knows. However, as we go to the philosophers we all babble hurly-burly about what ought to be done and what ought not, good and evil, fair and foul, and on these grounds assign praise and blame, censure and reprehension, passing judgement on fair and foul practices, and discriminating between them. But what do we go to the philosophers for? To learn what we do not think we know. And what is that? General principles. For some of us want to learn what the philosopliers are saying, thinking it will be witty and shrewd, others, because they wish to profit thereby. But it is absurd to think that when a man wishes to learn one thing he will actually learn something else, or, in short, that a man will make progress in anything without learning it. But the multitude are under the same misapprehension as was Theopompus, the orator, who actually censures Plato for wishing to define every term. Well, what does he say? "Did none of us before your time ever use the words 'good' or 'just'? Or, without understanding what each of these terms severally mean, did we merely utter them as vague and empty sounds?" Why, who tells you, Theopompus, that we did not have a natural conception of each term, that is, a preconceived idea of it? But it is impossible to adjust our preconceived ideas to the appropriate facts without having first systematized them and having raised precisely this question—what particular fact is to be classified under each preconception. Suppose, for example, that you make the same sort of remark to the physicians: "Why, who among us did not use terms 'healthy' and 'diseased' before Hippocrates was born? Or were we merely making an empty noise with these sounds?" For, of course, we have a certain preconception of the idea "healthy." But we are unable to apply it. That is why one person says, "Keep abstaining from food," and another, "Give nourishment"; again, one says, "Cut a vein," and another says, "Use the cupping-glass." What is the reason? Is it really anything but the fact that a person is unable properly to apply the preconceived idea of "healthy" to the specific instances?