“It will not be possible for the same thing to be and not to be.”
Aristotle, Metaphysics (W.D. Ross, 1984)
Not to be confused with Shakespeare’s famous lines (“To be, or not to be, that is the question”), Aristotle’s statement above is known as the Principle of Identity. This principle tells us that each thing has all the same characteristics as itself (if treated at the same time and in the same fashion).
Why is this principle so crucial to logic? To start, it allows us to determine when two terms refer to equivalent categories. By this, we mean when the referents of the terms – the objects/categories which a term picks out– have all the same characteristics. In addition, the identity principle allows us to determine when two terms are not equivalent–i.e. when the referents of the terms do not have all the same characteristics.
Knowing whether terms are equivalent or not is, in turn, crucial to building up a system of formal logic (which will become clear, should you choose to take more advanced courses in logic) . Furthermore, the use of the same term in non-equivalent ways within a given argument is what leads to the fallacy of equivocation (discussed in a previous lesson).