I remember reading a lot about sense perception as an undergraduate but not much since then. What follows might be hopelessly naive, therefore, but perhaps every blog post should be taken to be not yet fully worked out. Anyway, here goes.
It seems to me that there is a sense in which seeing is believing. Not to believe what you (think you) see involves an act of telling oneself that it is not real, which takes some conscious effort. And what is real is often determined by the senses, so that if something looks like one thing but is really another this means that, while to the eyes it may be a duck (or whatever), to the other senses (or to the eyes at another time, or from another angle, or in a different light) it is clearly a wooden model (or whatever). There is, then, a logical or grammatical connection between reality and sense experience: 'real' means something about sense experience.
Other things get called real too, but there is often controversy or mystery about their reality (are numbers real?, are universals?, is love?, etc.) and the claim that they are real usually (always?) comes back to our experience and making sense of it as best we can. But experience is not all sense experience. I mentioned love, which is not one simple feeling but involves various feelings, and pain is another well known example. There is no simple way to check for reality in the case of these experiences.
Take pain, for instance. If someone complains of being in pain then we usually believe them, but not always. If they have a wound then we would probably stop doubting. But in many cases we have to decide on very little evidence whether someone is malingering or not, insane or not, exaggerating or not. Sometimes we even have to decide (or might wonder, anyway) whether our own experience counts as pain or not. Feelings are not the kind of things you can fix clearly in your inner sights, as it were (at least not always); they can be very indistinct, fleeting, in flux. Another question is knowing what the right word for any particular feeling might be. What do other people call this? Or, given that there isn't really a 'this', just a sort of sea-surface of subjectivity, Do I have the right to call this 'pain' (or 'love' or whatever)? Should I suck it up and get back to work? Should I stop talking about it? Will this feeling last or fade away? There is no knowing--you have to make a decision, or at least act. You go back to work or you take to your bed, etc.
Sometimes these experiences are symptoms of something else, giving you reason to draw a kind of conclusion. I have pain in my arm, therefore maybe I'm having a heart attack. That kind of thing. But pain is certainly not always evidence of any problem beyond the pain itself. You have a headache, you cannot doubt that you have a headache (without denying your own sanity to the extent that you abandon all hope of rational thought), but you have no reason to conclude that you therefore might have a head wound. In the case of love, you might doubt whether the feeling is really love (or is it a crush?, or lust?, or is there even any such thing as true love?, etc.), but you cannot seriously doubt that you have some strong feeling. And it isn't evidence of anything, i.e. of the existence of anything outside of you.
When it comes to religious experience (and here I am getting to my point at last), people seem to want to treat it on the model of sensory experience. It is often said that there is an argument for the existence of God that goes: I (and/or others) have had an experience as of God, therefore God (probably) exists. It is then pointed out what a terrible argument this is, because it does nothing to prove that these experiences are not hallucinations or something of the sort.
But this objection treats the argument as if it were a claim to a kind of sense experience, which would then make God the kind of thing, a content of the world, that he is not meant to be. So it might seem better to treat religious experience as more like the experience of pain or love. The problem then is how this gives anyone a reason to believe in the existence of something outside the subject. And the answer to that would come back to our need to make sense of our experiences. Grammatically, feelings of love have an object (while feelings of pain do not). Love is of this or that person or thing. Pain is in something, but is not intentional, is not of or for or about anything.
I'm not sure there is a grammar of religious experience, but (if there is) it's more like love than pain. If you've had the experience you cannot doubt that you have had it. And it is typically an experience as of God (or "the One"). You can write it off as something less meaningful than it seemed at the time, or as something like a hallucination, but this takes work. Whether such effort is worthwhile depends on what best enables you to make sense of your experiences as a whole. And that is going to vary from person to person and over time, as experiences come and go.
Which might not be much of a thesis, but is an attempt to explain why I'm dissatisfied with standard treatments of the argument from religious experience. It isn't as bad an argument as non-theists typically think, but it also isn't anything like a rational proof that everyone should accept.