We see things as we are, not as they are.
-Leo Rosten
That may not be the right attribution; I see in Google Books attributions to Immanuel Kant, the Talmud, Adias Ninn, Halsey P. Taylor, and Preston James. I suspect one of the non-Rosten attributions is correct - the earliest Rosten hit from 1978 has the first use of the sentence in quotes.
A monthly thread for posting any interesting rationality-related quotes you've seen recently on the Internet, or had stored in your quotesfile for ages.