 A central question in philosophy is this. Can we know anything with perfect certainty? Throughout history, philosophers have disagreed on the answer. One popular response is to say, well, we can't ever be completely certain there's a sliding scale of certainty. Things aren't black and white, but rather they're 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 that 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 exist, but they 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 the confidence and the accuracy of any given proposition. Let me give you some examples. Number one, a low degree of confidence. One bitcoin will be worth more than $400 by the end of 2016. Now I am sold on bitcoin technology. I've written a book on the topic and you can listen to the audiobook for free on YouTube, but it's unclear what the future price will be at any given time. Essentially, this proposition is a hunch. Number two, a 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 that we don't have about the human body, I wouldn't bet the farm on it. Number three, a high degree of confidence. The sun will rise tomorrow. Every single time that I've awoken, the sun has arisen. I have no experience otherwise, nor does anybody else that I've ever spoken with. Now, it doesn't mean that I know with perfect certainty the sun will rise tomorrow. I mean, it could explode this evening, but I'm highly confident that it will rise. Number four, perfect certainty. Perception is a real phenomena. I am 100% sure that perception is a real phenomena in the universe. It happens. I know it because I experience it directly. I can't know that anybody else experiences perception, nor can I know that my perceptions are accurate. I mean, 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 is not a tautological truth. It isn't self-evident based on the construction of the sentence. It's not true by definition. It's not empty, nor is it a logical necessity. It could be otherwise. Instead, 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 phenomena in the universe. It's not because I'm naive, but because I have direct knowledge of the answer. You could also rephrase the sentence to say something like experience is happening. Well, how can I know experience is happening? Well, by being aware of it. If it's there at all, it's happening. Now, if what I've said is true, then the following is also true. Anybody who denies that perception is a real phenomena is wrong, and I know it with 100 percent certainty. I know 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. 2 plus 2 equals 4. Not every proposition is contradictory and non-contradictory at the same time. All of these sentences can be known to be true with certainty when you think about them, and you can also know by extension that their negation is false. The list of known truths might be relatively small compared to other larger bodies of knowledge, but it's still there, and it only takes a bit of imagination to discover. In fact, I would argue that certain knowledge underpins all of our other knowledge. Necessary, inescapable truths are presupposed by every sensible proposition, though they are often overlooked. So, I agree that we should recognize that there's a sliding scale of certainty, but we must recognize that it slides all the way to perfect 100 percent black and white absolute certainty.