The First Principles are undeniable or reducible to the undeniable

The First Principles are undeniable or reducible to the undeniable. They are either self-evident or reducible to the self-evident. And self-evident principles are either true by their nature or undeniable because the predicate is reducible to the subject. That the predicate is reducible to the subject means that one cannot deny the principle without using it. For example, the principle of non-contradiction cannot be denied without using it in the very denial. The statement: "Opposites cannot be true" assumes that the opposite of that statements cannot be true.
Not all skeptics or agnostics are willing to grant that the principle of causality, which is crucial in all cosmological arguments for God, is an undeniable first principle. Indeed, not every skeptic is willing to admit that something exists (the principle of existence). Thus,it is necessary to comment on their undeniability.