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You can't know what you don't know. You can't know about things you have yet to discover.

Since we cannot know all that there is to be known about anything, we ought to know a little about everything.

Knowledge is knowing that we cannot know.

There are things known and there are things unknown, and in between are the doors of perception.

As if one could know the good a person is capable of, when one doesn't know the bad he might do.

If a thing be really good, it can be shown to be such.

To be absolutely certain about something, one must know everything or nothing about it.

Knowledge is invariably a matter of degree: you cannot put your finger upon even the simplest datum and say this we know.

To explain the unknown by the known is a logical procedure; to explain the known by the unknown is a form of theological lunacy.