The law of noncontradiction states that nothing can both be and not be at the same time and in the same relationship. Put more formally, “A” can not be “B” at the same time and in the same relationship. The first part of the law is pretty straight forward. A fish can not be a cat or a frog can not be a table, at least at the same time. Being can not be nonbeing at the same time. Opposites can not be true at the same time and in the same relationship. It is contradictory to say that nonbeing is being at the same time. Contradictories can not both be true. One must be false, while the other is true.

Sometimes, however, the relationship part of the law is not understood. For example, I can be both a father and a son at the same time but not in the same relationship. I am my father’s son, and my son’s father but those are different relationships. Further, I can be my son’s biological father but not my son’s legal father if he were to be legally adopted. That, too, is a different relationship.

A basic feature of reality is that opposites can not exist in the same way and same relationship.

Because the law of noncontradiction is a basic property of being (reality), we learn that it is foundational to a proper understanding of metaphysics, ontology, and epistemology (theory of knowledge). Truth is clarified when the law on noncontradiction is properly understood.

The law of noncontradiction is a metaphysical first principle because it speaks to basic nature of reality. It helps us understand what is or is not so.

The law of noncontradiction is an ontological first principle because it points to the act or type of being a thing is–what can or can not be.

The law of noncontradiction is an empistemological first principle because it tells us what can or can not be known.

Finally, the law of noncontradiction helps us determine what is true and false, because opposites can not both be true. At the most fundamental level one statement or act of being can not be both true and false at the same time and same relationship.