To know that we know what we know, and to know that we do not know what we do not know, that is true knowledge.

To know that we know what we know, and to know that we do not know what we do not know, that is true knowledge.

Let no one expect anything of certainty from astronomy, lest if anyone take as true that which has been constructed for another use, he go away...bigger fool than when he came to it.

First of all, we must note that the universe is spherical.

Of all things visible, the highest is the heaven of the fixed stars.

At rest, however, in the middle of everything is the sun.