One of the great questions of all times is whether human beings can know any
absolute truth.
And a simple way to answer in the affirmative is to consider the principle of non-contradiction.
The ancient Greeks developed a number of disciplines or branches of knowledge
such as geometry, medicine, physics, et cetera.
In the language of Thomas Aquinas each of these branches of knowledge is a science,
and each science studies a different kind of reality.
Geometry is the study of continuous quantities, medicine the study of health, and physics
the study of moving things.
If each science studies some specific kind of reality or being, could there be a
discipline or science that does not study one specific kind of being, such as living
things or human beings, but studies being as such?
Aristotle thought so, and he called this science metaphysics.
The purpose of metaphysics is to ask about reality as a whole and to search out the features
and causes of beings as such.
Now, just as geometry has its principles, so too metaphysics has its principles.
One of the most important principles of metaphysics is the principle of non-contradiction.
In one of its standard forms the principle goes like this, it is impossible
for something to be and not be at the same time and in the same respect.
For example, it is impossible for a man to be both six feet tall and not six feet tall
at the same time.
It is impossible for a ball to be red and not red at the same time and in the same respect,
for example, all over.
Or for a simpler example still, it is impossible for Socrates to exist and not exist at the
same time.
Aristotle says that the principle of non-contradiction is a first principle.
It cannot be proven but also it doesn't need to be proven.
It is self-evident to reason.
It is a common principle or an axiom underlying all disciplines and all investigations.
If detectives working on a case, for example, were to find a set of clues that
a suspect was at a certain store at 7:00 PM on Friday night, and were later to find another
set of clues that the suspect was not at the store at 7:00 PM on Friday night, the detectives
would think through the clues in order to reconcile them.
But they would not hold that the suspect was both at the store at 7:00 PM Friday and not
at the store at 7:00 PM Friday.
No one would believe them, and they themselves would not be able to believe it, for in the
back of all our minds, we all know that it is impossible for something to be and not
to be at the same time and in the same respect.
No one can doubt the principle of non-contradiction once they understand
it, but many people misunderstand it at first glance.
Some people think it is saying that no human being can hold contradictory beliefs, but
that is not what the principal is saying.
Some people think it says that people cannot have conflicting experiences of things,
for example, the quality of a movie, but that is not what the principal is saying.
The principle is not about our subjective beliefs, experiences, or perspectives
on things.
The principle is about things, that is, reality itself.
It is saying that reality is not contradictory even though people often are, both within
themselves and with each other.
Since the principle of non-contradiction underlies all disciplines and all investigations we
can learn from it something important about reality as such:
Reality is self-consistent.
When we say that reality is self-consistent, we are not saying it about this or that kind
of thing, such as living things or quantities.
We're saying it about all things, about beings as such, and about reality as a whole.
The principle of non-contradiction stands as a kind of witness that human beings can
know something solid and true about reality as a whole.
The great calling of philosophy and metaphysics is to say much more about reality as a whole.
For readings, podcasts, and more videos like this go to Aquinas101.com.
While you're there, be sure to sign up for one of our free video courses on Aquinas.
And don't forget to like and share with your friends, because it matters what you think.