Is absolute certainty necessary for knowledge?

Many times when I am sitting at Ask an Atheist the idea that we need absolute certainty in order to have knowledge is brought up. It comes up whenever we are asked “how can you know God doesn’t exist?” Most of the time the answer that I hear given is that we don’t claim to know God doesn’t exist since most of us are agnostic atheists.  While this is technically correct I think it gives a little too much to the theist. If we except that knowledge does not require absolute certainty (which I’ll argue soon that we should) then I think we can claim that we do know that God doesn’t exist.

So why should we accept that knowledge does not require absolute certainty?  There are two arguments against both types of knowledge that we can have. The two types of knowledge being:

  • a priori
    • knowledge gathered prior to experience e.g. mathematical truths, laws of logic,
  • A posteriori
    • knowledge gathered through experience e.g. scientific knowledge, knowledge of the world around us

Both the arguments I will discuss come from René Descartes.  René Descartes position on what it requires to have knowledge is like most peoples prior to taking any philosophy classes infalliblism.  Infalliblism claims that we can only have knowledge of things of which we cannot doubt, this is another way of saying that we require absolute certainty in order to have knowledge.

The first argument from Decartes is what is known in philosophy as the dream argument.  The argument goes something like this: it is possible that we are in the dream that our entire life has been a dream (think of the matrix) and since there is no way to disapprove this prosperity e.g. we might be mistaken that it is actually the truth we have reason to doubt sense experience. This argument is then obviously an argument against a posteriori knowledge but Decartes reasoned that even within dreams your reasoning is the same in the dream state and the conscious state.  So this means that the dream argument cannot used against a prioi knowledge.

Does this mean that we can have certainty for a priori knowledge? Not quite to show why we must turn to Decartes other argument. This argument is called in philosophy the evil demon argument. This argument says that even ones reason could be fooled by a demon that wants to deceive you into believing false things. Since it is impossible to show that this demon does not exist we can doubt this type of knowledge. (By the way Decartes later goes on to use this as an argument for the existence of God by saying that well if we suppose that God exists and he is benevolent he would never allow us to be so deceived and so we can rely on reason because God exists).

I hope I have shown in this post that absolute certainty is basically impossible. And that if knowledge requires absolute certainty one is led into universal skepticism which means that we know nothing.

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