Severability
Profound ontological uncertainty writ large. If one aspect of my contract becomes somehow unenforceable, what does that mean for the rest of it?
A question that festers away in the minds of assiduous draftspeople the world over, but not one which often troubles the judiciary. This is really a way of looking at the question of illegality. The general proposition is that a legal contract the performance of which is illegal is void and unenforceable as a matter of pubic policy.
But what if only a teeny little bit of it is illegal?
These are the real world concerns to which modern lawyers turn their minds.