Knowing the Difference Between Contingent vs. Necessary Truths

The distinction between contingent and necessary statements is one of the oldest in philosophy. Truth is necessary if denying it would entail a contradiction. A truth is contingent, however, if it happens to be true but could have been false.

Example

Cats are mammals.
Cats are reptiles.
Cats have claws.

The first statement is a necessary truth because denying it, as with the second statement, results in a contradiction. Cats are, by definition, mammals, so saying that they are reptiles is a contradiction. The third statement is a contingent truth because it is possible that cats could have evolved without claws.

This is similar to the distinction between essential and accidental qualities. Being a mammal is part of a cat's essence, but having claws is an accident.