Cartesian skepticism
 
 
Any of a class of skeptical views against empirical knowledge based on the claim that claims to empirical knowledge are defeated by the possibility that we might be deceived insofar as we might be, for example, dreaming, hallucinating, deceived by demons, or brains in vats.
 

Details:
 
The gist of Cartesian-style skeptical arguments is that some empirical proposition (e.g. that there are trees) cannot be known because we might be deceived (e.g. we might be brains in vats hallucinating that there are trees). Related forms of these arguments attack our justification for believing some empirical proposition on grounds of possible deception. These 'justification' versions undermine claims to knowledge insofar as justification is a necessary condition on knowledge. The arguments I examine below are all of the 'knowledge' variety, but they can easily be transformed into arguments of the 'justification' variety by simply replacing all occurrences of 'knowledge' with 'justification'.
 
Here is a simple Cartesian-style skeptical argument:
 
A. I know that there are trees only if I know that I am not deceived that there are trees.
B. I do not know that I am not deceived that there are trees.
C. Therefore, I do not know that there are trees.
It seems pretty clear that this is a valid argument. But is it sound? Let us turn to inquire into the truth of the premises starting with premise A.
 
One possible argument in defense of premise A is the following:
 
1a. I know that there are trees only if I am not deceived that there are trees.
2a. If I know that p, and p entails q, then I know q.
A. Therefore, I know that there trees only if I know that I am not deceived that there are trees.
Again, we have a valid argument. But is it sound?
 
1a seems true enough: Knowledge of p entails truth of p and deception about p entails the falsity of p. Thus, I know p only if I am not deceived that p.
 
But 2a. is questionable. It is just a statement of the principle of closure of knowledge under entailment. It is called principle of closure under entailment because moving from something known to something entailed does not take us outside the closed area of knowledge.
 
But the principle of closure under entailment seems to be false. For example, someone can know that x is a triangle without knowing that the interior angle sum of x is 180 degrees, even though as a matter of fact, being a triangle entails having an interior angle sum of 180 degrees. If the principle of closure under entailment were true, then we wouldn't need to construct and examine valid proofs! Therefore, the second premise of the argument for A is false, rendering that argument unsound.
 
But maybe we can save premise A this way: instead of employing the principle of closure under entailment, we can employ the principle of closure under known entailment (i.e. If I know that p and I know that p entails q, then I know that q). This is called principle of closure under known entailment because moving from something known to something known to be entailed does not take us outside the closed area of knowledge.
 
This principle seems less controversial. For example, if someone knows that x is a triangle and knows that being a triangle entails that the interior angle sum is 180 degrees, then it seems that they must know that the interior angle sum of x is 180 degrees.
 
So now we can revise the defense of premise A, replacing 2a with 2a' and adding 3a yielding the following:
1a. I know that there are trees only if I am not deceived that there are trees.
2a'. If (I know that p and I know that p entails q), then (I know q).
3a. I know that (I know that there are trees only if I am not deceived that there are trees).
A. Therefore, I know that there are trees only if I know that I am not deceived that there are trees.
Note that in the revised defense of A we had to add a third premise. This kind of premise can be problematic for Cartesian skeptics that want to question empirical knowledge in general, since it itself is a piece of empirical knowledge. Nonetheless, it seems that the defense of premise A in the argument against knowledge of trees is in relatively good shape.
 
Let us now turn to examine premise B of the Cartesian skeptical argument. One defense of premise B is that I might be a brain in a vat. But maybe we can prove that I'm not a brain in a vat. Hilary Putnam, in the famous first chapter of his book Reason, Truth, and History, attempted to do just that. Putnam's anti-skeptical argument is as follows:
 
1. If I am a brain in a vat then I cannot think that I am a brain in a vat.
2. I can think that I am a brain in a vat.
3. Therefore, I am not a brain in a vat.
Again, the argument is valid. Is it sound? Well, it all seems to depend on premise 1. According to Putnam's brand of externalism, if I can think about vats, then I must have had appropriate causal contact with them. But if I am a brain in a vat, then I've never had appropriate causal contact with vats. Thus there is a problem for premise 1: Putnam's argument only works for brain in vat cases in which I (my brain) never was unvatted.
 
Let us suppose instead that I've lived a relatively normal life as an embodied brain, enjoying all sorts of normal causal contact with vats, brains, etc., until yesterday, when I was kidnapped in my sleep and turned into a hallucinating brain in a vat. It seems then that I could be a brain in a vat and think 'I am a brain in a vat'. And, if at the same time as my 'envattment', all the trees in the world were, unbeknownst to me, utterly destroyed, then my occurrent hallucination that there are trees in my backyard would be the utmost of deceptions. So, the brain in the vat defense of premise B seems to hold water (not to mention brains).
 
As we've seen, pretty strong defenses of the premises of the Cartesian skeptical argument can be given, thus the valid argument is, seemingly, also sound.
 
So much for skepticism about skepticism!
 
 
Pete Mandik