Hume gives an argument in the Enquiry Concerning Human Understanding gives an argument for skepticism about commonsense belief in an external world. Here is one reconstruction of this argument.

A=provable that sense perception is caused by external objects

B=provable that sense perception is not caused by the internal processes of the mind, dreams, hallucinations, virtual reality machines, etc.

X= a posteriori inference

Z= a priori inference

1. If A then B

2. If B then ((B by X) or (B by Z))

3. ~(B by X)

4. ~(B by Z)

C: ~A

Notice that the argument is only negative; it says that we cannot prove that the external world exists. The argument does not show that we disprove the hypothesis that the external world exists either.

This is too strong; someone who believes in the external world only needs to say that we have some reason to think that the external world exists, not that it can be proved without a doubt that external objects exist.

However, this maneuver will only work against the form of the argument I have presented. On this interpretation, Hume’s skepticism gets its bite by asking us to rule out things like dreams, hallucinations, etc (P1 above). One might say in response “I can’t rule out every one of these skeptical possibilities, but so what? All I have to have is a reason in favor of believing in the external world.”

But one could reformulate Hume’s argument, not to show that our knowledge of the external world is not certain (because of the skeptical alternatives), but to show that in fact we have no reason at all to believe in an external world.

This is where the way the argument is set up makes a difference. On the reconstruction given above, we could claim that we need not be certain to be warranted in our belief about the external world. On another view, it may not be possible to get any reason off the ground at all for thinking that the external world exists.


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