  According to Aristotle, first philosophy, or metaphysics, deals with ontology and first principles, of which the principle (or law) of non-contradiction is the firmest. Aristotle says that without the principle of non-contradiction we could not know anything that we do know. Presumably, we could not demarcate the subject matter of any of the special sciences, for example, biology or mathematics, and we would not be able to distinguish between what something is, for example a human being or a rabbit, and what it is like, for example pale or white. Aristotle’s own distinction between essence and accident would be impossible to draw, and the inability to draw distinctions in general would make rational discussion impossible. According to Aristotle, the principle of non-contradiction is a principle of scientific inquiry, reasoning and communication that we cannot do without.