Most epistemologists and Christian philosophers don't think that our ability to falsify the Cartesian deceiver should be a condition of knowledge. If we are unable to disprove the hypothetical Cartesian deceiver, that's not a good reason to doubt our beliefs. That doesn't cast reasonable doubt on our beliefs. Indeed, it would be unwarranted to take that thought-experiment too seriously.
Take internalism. Suppose you have introspective access to your reasons. They seem to be good reasons. But how is that a check against self-delusion? Like LSD, the Cartesian deceiver is persuading you to mistake bad reasons for good reasons. You can't help but find these reasons to be convincing, even though they are deceptive reasons.
Sensations are neither true nor false and so cannot function as premises by which our beliefs are inferentially justified.
I think that's too crude or overstated. There are different kinds of sensory information. The sound of breakers isn't true or false. But the spoken word (a sentence) can be true or false.
a) The spoken word is structured sensation that uses sound waves to encode and communicate ideas or propositions.
b) Likewise, although sensations alone are neither true nor false, sensory input, in combination with ideas, can generate true or false beliefs.
If I see a red rose, I can rightly infer that I saw a colored object. If every red object is a colored object, then that's a valid deduction.
Now, it may not be possible to derive the principle that every red object is a colored object from sensory perception or induction. That principle may be intuitive or innate. That must already be in mind for me to draw inferences about the rose. But seeing the rose, in combination with that a priori truism or analytical truth, yields a new and true belief.
I'm not so sure Steve and I are very far apart here, if at all. The first sentence of b) seems to capture the essence of my position. Our senses were designed to be secondary causes by which we form true beliefs. This causative process isn't arbitrary. It's not as if any old belief would normally be caused by a given sensation. Rather, sensations themselves are the product of interaction with our surrounding environment. That stimulus and our physical, divinely-created processing equipment yield non-arbitrary beliefs.
My only point was that sensations qua sensations aren’t logical justifiers. We can say spoken sentences are true or false, but I would consider that a kind of metonymy (or some such literary device) which substitutes, for the intended meaning of the physical expression or manifestation, the actual meaning, which isn't physical.
Finally, I had written:
Our senses can cause numerous false beliefs. Sense knowledge is fallible.
True, but the same can be said for reason and memory. Scripturalists need to get down from their high horse and join the rest of us at ground level. They stipulate an inhumane standard of knowledge. Finite creatures can't satisfy those godlike conditions. But why should we?
…if infallibilist, internalist justification is possible in principle, and all our beliefs rely on memory - or even just beliefs relevant to forming infallibly, internally justified beliefs - I would suppose a transcendental argument could be constructed to defend against the idea all memorial beliefs are fallible in respect to justification. This could be a way in which memorial and sensory beliefs are relevantly disanalogous. That's why I'm interested in whether the actual believing of a proposition always, in human cases, requires memory. But this is just a suspicion.