Indefinables are concepts so global that they cannot be defined; rather, in a sense, they themselves, and the objects to which they refer, define our reality and our ideas. Their meanings cannot be stated in a true definition, but their meanings can be referred to instead by being placed with their incomplete definitions in self-evident statements, the truth of which can be tested by whether or not it is impossible to think the opposite without a contradiction. Thus, the truth of indefinable concepts and propositions using them is entirely a matter of logic.