A central question in philosophy is “Can we know anything with perfect certainty?” Throughout history, philosophers have disagreed on the answer. One popular response is to say “We can’t ever be completely certain – there’s a sliding scale of certainty. Things aren’t black and white, but rather, different shades of gray.” The scale goes from not-so-sure to very-sure-but-not-completely.

This answer is humble. It seems reasonable to always keep an open mind and recognize the possibility of being wrong; new information might arise in the future and change our minds. In my own conversations, people with this perspective often view it as arrogant, naive, or dogmatic to think anything can be known with absolute certainty.

But they’re mistaken.

I love the idea of a sliding scale of certainty, as long as it slides to 100%. Shades of gray are book-ended by pure black and pure white.

We can indeed know things with absolute certainty. Other things, we can only have partial certainty, while others we can only guess at. By “certainty”, I mean our confidence in the accuracy of any given proposition. Let me give some examples:

Low degree of confidence: One bitcoin will be worth more than $400 by the end of 2016.

I am sold on the technology, but it’s unclear what the future price will be at any given time. Essentially, this proposition is a hunch.

Modest degree of confidence: Trans-fats are bad for human health.

I’ve done some research, and I’m pretty confident in concluding that trans-fats are bad for you. However, given the huge amount of knowledge we don’t have about the human body, I wouldn’t bet the farm on it.

High degree of confidence: The sun will rise tomorrow.

Every single time I’ve awoken, the sun has arisen. I have no experience otherwise, nor does anybody else I’ve spoken with. It doesn’t mean I know with perfect certainty that the sun will rise tomorrow – it could explode this evening – but I’m highly confident that it will rise.

Perfect certainty: Perception is a real phenomenon.

I am 100% that perception is a real phenomenon in the universe. It happens. I know this because I experience it directly. I can’t know anybody else experiences perception, nor can I know that my perceptions are accurate; I might be hallucinating, or I could be a brain-in-a-vat. But regardless of the accuracy of my perceptions – or the cause of them – they happen.

Notice, this isn’t a tautological truth. It isn’t self-evident based on the construction of the sentence. It’s not definitional. It’s not empty, nor is it a logical necessity – it could be otherwise. It is a claim about the nature of reality which you can know with perfect certainty. Nobody will ever be able to convince me that perception isn’t actually a phenomenon in the universe – not because I’m naive, but because I have direct knowledge of the answer.

You could also rephrase the sentence to “Experience is happening.” How can I know experience is happening? By being aware of it; if it’s there at all, it’s happening.

If what I’ve said is true, then the following is also true: anybody who denies that perception is a real phenomenon is wrong. I know with 100% certainty that the sentence “Perception does not happen” is false.

This example is not isolated. Consider several other propositions:

“Communication can be attempted.”

“I do not know every fact about reality.”

“Something exists.”

“Perception might be illusory.”

“Language is real.”

“Some claims about reality are false.”

“I doubt some things.”

“Two plus two equals four.”

“Not every proposition is contradictory and non-contradictory at the same time.”

All of these can be known with certainty, when you think about them. And you can also know their negation is false.

The list of known-truths might be small relative to larger bodies of knowledge, but it’s still there. It only takes a bit of imagination to discover. In fact, I would argue certain knowledge underpins all our other knowledge. Necessary, inescapable truths are presupposed by every sensible proposition, though they are often overlooked.

So, I agree that we should recognize the sliding scale of certainty. But we must recognize that it slides all the way to “perfect, 100%, black-and-white, absolute certainty.”