This whole spirit was specially incarnate in another of Euripides' teachers. We hear of Protagoras in his old age from that enemy of the sophists, Plato. But for this sophist even Plato's satire is kindly and almost reverent. Protagoras worked not at physical science, but at language and philosophy. He taught men to think and speak; he began the study of grammar by dividing sentences into four kinds, Optative, Interrogative, Indicative, Imperative. He taught rhetoric; he formulated the first theory of democracy. But it was as a sceptic that he struck men's imaginations most. "About the Gods, I have no means of knowing either that they are or are not. For the hindrances to knowledge are many, the darkness of the subject and the shortness of man's life." Numbers of people, no doubt, went as far as this, and without suffering for it as Protagoras did; but his scepticism cut deeper and raised questions still debated in modern thought. "Man is the measure of things"; there is no truth to be had beyond the impression made on a man's mind. When this given object seems one thing to A and another thing to B, it _is_ to each one exactly what it seems; just as honey not only seems sweet but _is_ sweet to a healthy man, and not only seems bitter but _is_ bitter to a man with jaundice. Then you can not say, we may ask, that one or other impression is false, and will prove false on further inquiry? No: he answers; each impression is equally true. The only difference is that each state of mind is not equally good. You cannot prove to the jaundiced man that his honey is sweet, for it is not: or to the drunkard that he does not desire his drink, for he does: what you can do is to alter the men's state of mind, to cure the jaundice or the drunkenness. Our cognition flows and changes. It is the result of an active impact upon a passive percipient. And, resulting from this change, there are in practice always two things to be said, a pro and a con. about every possible proposition. There is no general statement that cannot be contradicted.