Phenomenology - what it's like to be a language user ...

Traditional sensory empiricism is incoherent for a lot of reasons, but here's a particularly acute one:

If I have an empirical belief that I am particularly sure about, because I have had some experience which confirms it, the relationship between the belief and the experience must be of a certain kind - it isn't enough that the experience generates the belief. The way the experience generates the belief has to be convincing - it has to conform to our intuitions about the relationship between perceptions and what we conclude from them.

For instance: If I thought there was an ostrich in the room, and based my belief on certain sensory impressions, these impressions would have to have a certain kind of relationship with the presence of ostriches.  If I tried to describe the impressions to you, they might sound like the impressions of something a bit large, fluffy, and bad-tempered. They would be ostrich-like.

If I was wired up badly, so that when my foot was tickled I thought there was an ostrich present, I might be just as convinced as someone who had ostrich like sense impressions, but I would be much less convincing. I might say "I have an ostrich presence foot tickle".

Either that, or I would actually have ostrich like sense impressions when my foot was tickled.  Instead of wriggling and laughing, I would say 'I see something large, fluffy and angry'. I might discover, or be shown, that this had to do with my foot being tickled, but this would not make the sensation tickly - it would remain ostrich-like.

Now, suppose that, in the real world, I was hooked up to a machine in such a way that the presence of an ostrich would result in the ticklish sensation in my foot, but that I did not know this.  Now I have a sensory impression which is a reliable indication of the presence of ostriches, but has nothing ostrich-like about it. Even if the relationship between the tickle and the presence of ostriches was explained to me, I do not think I would be inclined to say that ostriches were tickly. At least not if I wanted to make any sense to someone who doubted my ostrich awareness capacities.

And what if we close the loop? What if I am hooked up to such a machine, but am also in the condition of having ostrich-like sensations when my foot is tickled? An interlocutor unaware of my physical circumstances (speaking to me on the phone, for instance) might find my ostrich description quite convincing, and be, correctly, persuaded that there was an ostrich in the room with me.

And this final case is, perhaps, the most realistic. We have no direct access to each others' perceptions, or to the machinery by which they are produced. We take for granted that this machinery is reliable because it makes us talk in ways which are intelligible - not because we either understand how it works or, further, whether that understanding could ever lead to epistemological insight.