Protagoras

Man is the measure of all things: of things that are, that they are, and of things that are not, that they are not.

Concerning the gods, I have no means of knowing whether they exist or not, nor of what sort they may be. For many are the obstacles that prevent knowledge, both the obscurity of the question and the shortness of human life.

There are two sides to every question, and each side is equally valid.

Education does not come into the soul from the outside, but is already in the soul, and only needs to be awakened.

Justice is what is advantageous to the stronger.

The art of rhetoric is to make the worse appear the better cause.

Virtue is teachable.

The human mind is the measure of all things.

Every man's opinion is true.

There is no absolute truth, only relative truth.

What seems to each man is true for him.

The wise man, when he has to deal with the affairs of the city, will make good things seem just to the city, and bad things seem unjust.

Man is the measure of all things, of those that are, that they are, and of those that are not, that they are not.

It is possible to make the weaker argument the stronger.

The gods are not to be known by human reason.

For every question, there are two opposing arguments.

The best way to live is to live well.

All things are relative.

There is no objective truth.

Knowledge is perception.