Robin asked:

What is the common sense reply to Descartes method of doubt?

Reply by Peter Jones

Descartes’ method of doubt is just that, a method. In order to discover what we know and what we don’t know we identify which of our beliefs we can doubt and which we cannot. As Solipsism is unfalsifiable there is very little that we cannot doubt. If the external world can be doubted, then whatever it is that we cannot doubt can only be a knowledge of ourselves, of our own awareness and identity. In this case any reliable world-theory must be derived from an axiom stating self-knowledge.

This is not an optional method. Nor is It is a new discovery of Descartes’. Aristotle has in mind the possibility of doubt when he writes ‘True knowledge is identical with its object’. If there is no identity, then there is the possibility of doubt. There is therefore no common sense reply to Descartes’ method, we can only choose to use it or not. Common sense would say that we must.

In order to build a systematic theory of the world from the ground up on sound logical principles we must begin with an axiom. This may be an assumption or conjecture, but ideally it would be a certain fact. But what is a certain fact? Cogito was Descartes’ answer, the fact that he was aware he was thinking.

It may be possible to doubt that cogito is a certain fact, since it may be a false reification of a distinct ‘I’ that is thinking, but it is not possible to fault Descartes’ initial approach and method. If we do not take the trouble to establish what can be doubted, then we cannot be sure what we know and what we don’t, and this is obviously not a sound basis for building a world-theory.


Answer by Geoffrey Klempner

Probably the most famous example of the ‘common sense’ reply to Descartes is in the paper, ‘Proof of An External World’ by the British Philosopher G.E. Moore

“I can prove now, for instance, that two human hands exist. How? By holding up my two hands, and saying, as I make a certain gesture with the right hand, ‘Here is one hand’, and adding, as I make a certain gesture with the left, ‘and here is another’. And if, by doing this, I have proved ipse facto the existence of external things, you will all see that I can also do it now in numbers of other ways: there is no need to multiply examples.”

I discuss this ‘proof’ in my YouTube video, ‘Why am I here?’ To me, this looks like a prime example of a philosopher ‘reciting a magic spell’.

There was a time (which is not now) when the idea that you could base philosophy on ‘plain common sense’ seemed to be a viable option. In fact, the idea has appeared more than once in the history of philosophy. The problem is that, although we ultlimately want to defend common sense (there really is a world out there) the way to do it requires more than simply asserting, ‘This is what I believe and I refuse to consider the possibility that I might be wrong’.