Custom, then, is the great guide of human life.

Truth springs from argument amongst friends.

Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them.

You must learn all things: both the steadfast heart of well-rounded Truth and the opinions of mortals, in which there is no true trust.

-- Parmenides

For thinking and Being are the same.

-- Parmenides

Color exists by convention, sweet by convention, bitter by convention; in reality there are only atoms and void.

-- Democritus

I know that I know nothing.

-- Socrates

Imagine prisoners chained underground since childhood, seeing only shadows on a wall and taking them for reality.

-- Plato

Things are equally indifferent, unstable, indeterminate.

-- Pyrrho