The idea is that if a person does not have established principles, he or she is vulnerable to the blandishments of whatever philosophy encountered.

I think it's "If you don't stand for something, you will fall for anything" and it means: If you don't stand for something - if you don't believe in something, if you have no convictions of your own to begin with,.. you will fall for anything - you will easily be made to follow ideology that is less than worthy, you will easily fall for just about anything.