FIRST ARGUMENT FOR INTERNALISMOf course, this argument depends on the first premise being true, which means that a person already has to accept what is called deontological justification for it to be true. But like I said in a previous post, deontological justification could be true, non-deontological justification could be true, or they could both be true--or neither. Given the controversy about this premise, perhaps another argument could be given. And so:
1. Reasons for a belief are a matter of not being obligated not to believe them (deontological justification).
2. Not being obligated not to believe something is an internal matter.
3. So, reasons for a belief are an internal matter.
SECOND ARGUMENT FOR INTERNALISMFor this argument to be true, the third premise must be true, the premise about mentalism. However, unless someone is willing to accept that internal consistency is all that matters, they are unwilling to accept internalism as providing good reasons for a belief. In fact, this is the very thing the debate is about!
1. Suppose Billie and Billie* exist.
2. Billie and Billie* have an identical set of beliefs.
3. The consistency of mental states is what matters for having good reasons for a belief (mentalism).
3. So, Billie and Billie* have good reasons for their beliefs and they are internal.
A third way to be an internalist would be to accept evidentialism, which is supposed to be evidence that is discovered and is internal to a person. I don't quite understand this position still, since any philosopher who argues this way is using evidence idiosyncratically, it seems. Evidence seems to be, in ordinary language, a matter of something "out there". By definition, it isn't that, according to the way the philosophers are using it.
Anyway, again, even though we aspire to make the good reasons for our beliefs a matter of factors external to a thoughts and percepts, we nonetheless have to use those thoughts and percepts to discover what we discover about the world. So, the answer seems clear to me. We must be, as a matter of method and fact, internalists but we aspire to, although it is impossible to achieve, externalism. The problem, however, is that externalism is not something can be arrived at since we can never get outside ourselves.