The Law of Self Contradiction (sometimes referred to as simply "The Law of Contradiction") is the foundation of logic, but even beyond that, it is the most basic, fundamental premise upon which everything else must stand. It is even more fundamental than Cogito Ergo Sum, and it is unique in that the very act of doubting this law confirms it. The very act of even trying to build the foundation on anything other than this also confirms it, and places it at the foundation of whatever you are trying to build, or deconstruct, or do anything at all. It is stated like this:
Two antithetical propositions cannot both be true at the same time and in the same sense.
To make it simpler to understand, a concept may not both be, and not be, within the same context. It can't be true and not true at the same time, in the same world, in the same situation.

This says that true is not false. That 1 ≠ 0. That up is not down, and left is not right. The "within the same context" part is important though, because left can be right, depending on the current perspective at the moment. But within the same context, the same perspective, the same set of conditions, a thing is a thing and only that thing and not anything else.

What happens if you try to disbelieve or doubt this statement? Well, if a concept can be true and false at the same time, by declaring this false, you declare it true.

What happens if you try to ignore it before you establish anything else? The "anything else" is suddenly rendered false (and true) until the law of self contradiction is rendered true, which is immediately.

You can't get simpler than this folks. This is the basis for everything it is possible to know as a human being. It is hardwired into us, it is so fundamental that hardly anyone thinks about it. You don't have to think about it. You already know it, and have always known it. It just is. And that's the way things go.