A Priori Justification: Defeasibility

I want this post to be a discussion about something of which I am unsure of. I want to know whether there can be a hierarchy of defeaters.

Let me give you a background story, I was discussing a priori justification with a group of my peers and we were talking about the fact that some experiences can defeat a belief you hold by some other experience (remembering you left the keys on the kitchen table only to find out you didn’t leave them on the table when you actually go to look for them…etc.). One of my peers argued that this “inconsistency” amongst experience based beliefs is a problem thus his formulation of a priori justification leaves out even the possibility of being defeated by experience.

I, on the other hand, argued there is a sense in which we have a hierarchy of what beliefs defeat others. I proposed that direct sensual perception will end up defeating all other experience based beliefs (testimony, memory, etc.). I even offered an example of experience defeating a possible a priori belief. The example I offered was of Hesperus and Phosphorus (the evening and morning star). Now, it was the belief of ancient astronomers that this one star was actually two stars. So, depending on how you justify a belief a priori, it seems that one can say that it is a priori justified to believe that one star can exist without the other (the morning star existing while the evening star does not). It would be analytic and it would be a necessary truth that one can exist without the other so it would a be hopeful candidate for being justified a priori according to those two non-epistemic conditions (which are quite popular in epistemology). But, it seems that if one actually flies into space and just looks at the star and realizes it is both the morning and evening star then one might say the belief that one star can exist without the other has now been defeated by experience.

In response I was offered an example of a person who holds the ability to see two completely different visual fields and not a blend of both eyes. It could be the case that if this person’s eyes malfunctioned they may see something being in two places at once (or some sort of contradiction of temporal objects) if you were to see some sort of contradiction occur, my peer asserts, you would not belief it. Instead you would realize by the logical law of noncontradiction that the belief is false. Since logical laws are a good candidates for being a priori knowledge it may be the case that an a priori belief could defeat the belief justified by experience.

So does the hierarchy of potential defeaters put experience at top? Or does it put a priori beliefs at the top?

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