The Answer:

That saying was taken from a soliloquy by Polonius in Act I, Scene 3 of William Shakespeare's Hamlet. Polonius is giving advice to his son Laertes before Laertes heads back to school. Here is more of the quote.

Neither a borrower nor a lender be;      
For loan oft loses both itself and friend,      
And borrowing dulls the edge of husbandry.      
This above all: to thine own self be true,      
And it must follow, as the night the day,      
Thou canst not then be false to any man.

Some people may say that this is the advice of a fool. But it certainly sounds nice, doesn't it?

If you are interested in learning more about Hamlet, you might like Lynch Multimedia's prose adaptation. In particular, their classroom version places a modern prose version alongside the original text, for ease of comparison.

—The Editors