Many philosophers have thought that the following proposition wasa priori:
- If I don’t have any positive evidence that I am being deceived by any Cartesian evil demons, I am not being deceived by any such demon.
But how could this be a priori? Here’s a suggestion, which I’ll call the a priori bootstrapping argument.
Suppose that the following principle of rational belief is true: If one has a conscious experience as of its being the case that p, and no special strong reasons for regarding one’s current experience as unreliable with respect to propositions like the proposition that p, it is rational to form the belief that p. For short, let say that according to this principle, it is rational to “take experience at face value, in the absence of defeaters.”
Let us also suppose that there are some other similar principles of rational belief: a principle according to which it is rational to form introspective beliefs in which we self-ascribe our current conscious experiences; a principle according to which it is rational to form beliefs in the obvious logical consequences of propositions that we already rationally believe; and a principle according to which it is rational to form beliefs by means of inference to the best explanation.
Now imagine a Platonic soul waiting to beam down from the intelligible world into the sensible world. This Platonic soul knows all these principles of rational belief. So this soul can predict that so long as her experiences contain no defeaters, it will be rational for her to take her experience at face value. So she knows that so long as her experience contains no such defeaters, it will be rational for her to come to believe what Roger White, in his forthcoming paper “Problems for Dogmatism”, calls a “Track Record Proposition”, i.e. some proposition of the form:
- I have an experience as of its being the case that p1, and p1; I have an experience as of its being the case that p2, and p2; … and I have an experience as of its being the case that pn, and pn.
If it were rational to believe any such “Track Record Proposition”, it would surely also be rational to believe the following proposition (“No Errors”):
- The contents of my experiences have been true.
And if it were rational to believe that, surely it would also be rational to believe what the following proposition (“Reliability”) which seems to be the best and simplest explanation of “No Errors”:
- My experiences are generally reliable guides to the truth.
Moreover, we may presumably assume the Platonic soul knows that if it is rational for her to believe “Reliability”, it is also rational for her to believe that she is not being deceived by a Cartesian demon.
So the Platonic soul already knows – even before she’s beamed down to the sensible world – that if her experiences don’t contain any defeaters, it will be rational for her to believe that she isn’t being deceived by a demon. But then surely it will already be rational for her to believe that if her experiences don’t contain any such defeaters, she is not being deceived by a demon. (This would follow by means of a principle like Roger White’s “meta-justification principle” – i.e. roughly, the principle that evidence that one will acquire evidence that p is already evidence that p.) If it is already rational for the Platonic soul to believe this, even before she beams down into the sensible world, then it seems that her justification for believing this is a priori.