An Argument for Epistemic Skepticism
Consider this argument for a non-global epistemic skepticism. (I set aside global or total skepticism.)
Take any proposition p regarding the observable physical world, say, that there is a tree in the yard.
If you know that p, then you know that there is no evil demon deceiving you into believing falsely that p.
You do not know that there is no evil demon deceiving you into believing falsely that p.
Therefore, you do not know that p.
This is an interesting argument. Versions of it have been around for a long time. The argument is inspired by Descartes’ “evil demon” thought experiment. Peter Unger articulates a version in Chapter 1 of Ignorance: A Case for Scepticism. The Matrix films are based on a similar idea, namely, that one is a brain (or a brain and body) in a vat, that one’s experiences are generated and controlled by the computerized matrix, and hence that one’s experiences — although real — are not what they seem to be.
The argument can be construed differently depending on one’s assumed definition of ‘propositional knowledge.’ Let’s grant (1) arguendo. Now, suppose you take propositional knowledge to be fallibly justified true belief. In that case, it is reasonable to deny (2); in other words, one does in fact know (i.e., believe on fallible justification) the true proposition that there is no evil demon deceiving you into believing falsely that p – assuming that it is true that no demon is deceiving you (if that proposition is false, then one does not know after all). One can thus turn the modus tollens into a modus ponens:
If you know that p, then you know that there is no evil demon deceiving you into believing falsely that p.
4. You know that p.
5. Thus, you know that there is no evil demon deceiving you into believing falsely that p.
But suppose instead you take propositional knowledge to be infallibly justified true belief. That is, you are an infallibilist about epistemic justification. You hold that knowledge requires epistemic certainty (i.e., if one knows that p, then one’s belief that p cannot be false given one’s evidence for p.) In this case, it is reasonable to affirm (2) and thus to conclude that (3).
In this sense, then, the case for skepticism turns on the definition of ‘propositional knowledge,’ which in turn depends on the nature of epistemic justification.