Shakespeare Interpretation. There's a line in Hamlet that everyone knows, that goes like this...

This above all: To thine own self be true

The problem is that this is only part of the line. And most people interpret it to mean... Be true (i.e. Loyal) to yourself.
HOWEVER... the real line goes like this:

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.

So what he's really saying is "don't lie to yourself... tell the truth to yourself... and the natural progression will be that you will then be truthful to everyone."