It is funny where the lessons of our classrooms are sometimes understood.

I studied philosophy when I was still a bit too young for it, a 19-year-old studying the metaphysics of Aristotle and St. Thomas Aquinas. But something from a metaphysics course remains indelibly stamped in my mind.

We learned that there are four “transcendental” properties to God: Scholastic metaphysics tells us that God is one, true, good, and beautiful. My young mind then had some grasp of what is meant by true, good, and beautiful since we have some common sense notions of what these are; but what is oneness? What is divine about being undivided?

The answer to that didn’t come to me in a classroom or in an academic discussion, even though I have often tried to explain its meaning to students in a classroom. It came to me in a grocery store.

I had been buying groceries in the same store for 12 years when a trivial incident helped explain God’s oneness and its importance to me. The store, a large supermarket, has a fruit aisle where you pick up apples, oranges, grapefruits, bananas, and the like and then bag them yourself in plastic bags the store supplies.

Alongside the plastic bag dispensers there are small containers holding metal twisters you use to tie up the top of your bag. One day, I picked up some fruit, put it into a bag, but all the containers containing the twisters were empty, every one of them.

As I checked out my groceries, suspecting that possibly someone had taken them as a prank, I mentioned to the cashier that all the twisters were gone. Her answer took me aback: “But, sir, we have never had them in this store!”

Thinking she might be new on the job, I said, “I’ve been coming here for more than 10 years and you’ve always had them! You can even see their containers from here!” With an assurance that comes from absolute certitude, she replied, “I’ve been working here for a long time, and I can assure you we’ve never had them!”

I pushed things no further, but, walking out of the store I thought this to myself: “If she’s right, then I’m certifiably insane! If she’s right then I’m completely out of touch with reality, have been for a long time, and I have no idea what sanity is!” I was certain that I had seen the twisters for 10 years!

Well, they had reappeared by the next time I entered the store and they are there today, but that little episodic challenge to my sanity taught me something: I now know what it means that God is one and why that is important.

That God is one (and not divided) is the very foundation for all rationality and sanity. That God is undivided and consistent within assures you that two plus two will always be four, and that you can anchor your sanity on that. That God is undivided assures you that if you saw package twisters in a store for 12 years, they were there, and you are not insane.

That God is one is the basis for our sanity. It undergirds the principle of noncontradiction: Something is or it is not, it cannot be both; and two plus two can never be five, and that allows us to live rational, sane lives. Because God is undivided, we can trust our sanity.

The truth of this was never jeopardized by the great epistemological debates in history. Doubts about rationality and sanity do not come from Descartes, Kant, Hegel, Locke, Hume, Wittgenstein, or Jacques Derrida; these philosophers merely argued about the structure of rationality, never about its existence.

What jeopardizes our sanity (and is, no doubt, the greatest moral threat in our world today) is lying, the denial of facts, the changing of facts, and the creation of fake facts. Nothing, absolutely nothing, is as dangerous and pernicious as lying, dishonesty.

It is no accident that Christianity names Satan the Prince of Lies and teaches that lying is at the root of the unforgivable sin against the Holy Spirit. When facts are no longer facts then our very sanity is under siege because lying corrupts the basis for rationality.

God is one! That means that there is no internal contradiction within God and that assures us that there is no internal contradiction possible within the structure of reality and within a sane mind. What has happened, has forever happened, and cannot be denied. Two plus two will forever be four and because of that we can remain sane and trust reality enough to live coherent lives.

The single most dangerous thing in the whole world is lying, dishonesty, denying facts. To deny a fact is not only to play fast and loose with your own sanity and the very foundations of rationality; it is also to play fast and loose with God whose consistency undergirds all sanity and all meaning. God is one, undivided, consistent.