Renee Descartes was the first modern philosopher. He was a rationalist. His goal was to ground knowledge in something that could not be doubted. He found such a grounding in his famous formulation, Cogito, ergo sum (I think, therefore I am). The question to be answered was how he could know he existed. The answer was to be found in his act of contemplation of the very question. To contemplate existence requires a contemplator who exists. That he was thinking about doubt was something he could not rationally doubt, and thus concluded he knows indubitably that he exists. He reasoned deductively as follows:

P1 The act of thinking requires the existence of a thinker
P2 I experience the act of thinking
______________________________________________
I exist as a thinker

Some argue that Descartes key insight actually turns out to depend on a logical fallacy: begging the question. The question is whether there exists a personal subject, “I.” And yet “I” is smuggled into the second premise of the argument. That is question-begging, for it assumes there is an I to experience the act of thinking, and then concludes that there is an I who thinks. I am conflicted about this. On the one hand, this seems reasonable to me. Descartes reasoning does seem to beg the question. On the other hand, Descartes argument seems valid: the ability to contemplate one’s existence requires that they exist. What do you think?

If Descartes did beg the question, invalidating his argument, then it seems there is no non-question-begging argument that could indubitably prove I exist. Of course, this does not mean I do not exist. I do, and I know I do. It simply means we can’t demonstrate how we know this, other than an appeal to basic intuition.

I think this is a helpful lesson for skeptics. One does not need to be able to prove (or know how) X is true in order to know X is true. Some truths are properly basic; i.e. they are self-evident, do not need to be questioned, and do not need evidential demonstration.

 

UPDATE 3/1/17: Perhaps the supposed question-begging nature of the argument is merely the fault of how analytic philosophers structure the argument. For example, if we state the argument as follows, it does not beg the question:

P1 The act of thinking requires the existence of a thinker
P2 There are acts of thinking ______________________________________________
Thinkers must exist