Why everyone must deny the difference principle

Suppose that you believe that some part of reality is necessary, whatever that part might be, and some part of it is contingent, whatever it might be. Whatever is necessary, is and could not be otherwise. Whatever is contingent, is but could be otherwise or could even fail to be at all. Call the contingent aspect of reality C and the necessary aspect of reality N.

What is the relation that obtains between C and N?

Either C has an efficient cause or it does not.

If it does not, then it is a contingent reality which exists uncaused, a brute fact in violation of the Principle of Sufficient Reason or of any other such principle of metaphysics.

If it has an efficient cause, then it either efficiently causes itself or else is efficiently caused by N. Presumably it cannot cause itself, nor could it be caused by something other than what is included within N and C. Beyond the necessary and the contingent, there is only the impossible.

If N is the efficient cause of C, then the efficient causality of N either obeys the difference principle or it does not.

The difference principle states that a possible difference in effect presupposes a possible difference in the cause. If the cause cannot be otherwise, then its effect cannot be otherwise.

If the causality of N obeys the difference principle, then C as the effect of N cannot be otherwise than it is because N cannot be otherwise than it…