The good news is that this feature of after-images seems no more difficult to explain than how we see physical object like a table 'out there'. True, there is an actual table out there while there is nothing corresponding to the after image out there, but this makes no difference. In both cases, something inside our head is managing to present something as out there. (Fortunately, the table doesn't come into our heads when we see it.) So I think your question is just how the brain or the mind manages to see things as outside itself, whether there really are those things out there or not. The bad news is that we really seem to have no idea how this is possible (which is not to say that philosophers -- including notably Immanuel Kant -- haven't spent a lot of time worrying about it).

This is an excellent question, but one of the reasons it is not easy to answer is that we are not comparing like with like, because different senses give us information about different kinds of thing. It’s not like two people who tell you about astronomy where we might say that one person is more credible than the other on the same subject. Take sight and taste. Sight seems to give more information about the external world, but in part for that very reason it is more open to error. Taste seems to give much more restricted and subjective information, and in part for that very reason it is less open to error. Having said that, however, all the senses may form the basis for inferences about what is going on in the external world, and any of these inferences may go wrong. Another reason your question is hard is that philosophers don’t agree about what counts as the senses getting things right. Take the colors that we believe things to have on the basis of sight. Some philosophers think that...

I agree that sight and indeed the senses are our anchor to the physical realm, but I think that is a good thing, since the physical realm is what we need to understand. At the same time, our senses give us only very limited contact with the physical world, much of which is unobservable. Creatures with different senses might have a cognitive advantage over us in this respect. But any sort of physical sense we can imagine (such as echolocation in bats, or exotic sensory sensitivity to chemicals) would still require massive inference from the limited effects of the world the experiencer to the complex worldly causes of those effects. At the same time, science has done an impressive job of reducing the limitations stemming from the peculiarities of the human sensory endowment through the construction of instruments that enable us to detect things we cannot directly observe.

Sensations are real in my book while I am having them, but you are right that it is not easy to use them to build a table. The trouble is that the table has a continuous existence, while it is only intermittently observed. One standard way around this problem is to fill in the apparently unobserved periods by having God observe the table all the time. That was George Berkeley's suggestion. Another way is to fill in the gaps between actual table sensations with merely possible sensation. This too is suggested by Berkeley in one remarkable passage, and figures centrally in John Stuart Mill's phenomenalism, acccoring to which tables are, in his memorable phrase, 'permanent possibilities of sensation'. The idea is that even when nobody is looking at the table, it is still true that if someone were to come into the office, they would see the table. Phenomenalism is a clever solution to the gap problem, but like many other philosophers (not to mention ordinary people) I find it incredible to...

Perception is not ingestion: when you see a tree, the tree does not enter your brain. Moreover, seeing does involve the creation of an image on the backs of your eyes. But it does not follow that you only ever see an image. Maybe an analogy to a photograph will help. A photograph of a tree is an image; but it is not a photograph of an image, it is a photograph of a tree. Similarly, seeing a tree may involve an image of the tree, but what we see is the tree, not the image.

The popular dispositional theory of colour that Richard mentions has a curious consequence. If being red is just being such as to tend to produce a certain kind of sensation in us, then it isn't even possible that what tends to look red to us isn't really red but is really say some shade of gray. For on the dispositional view, red just is tending to look red.

There are three main options here that philosophers have developed for properties like sweetness: sweetness is a sensation, it is a disposition in some things to produce a sensation, or it is an intrinsic property of sweet things (presumably to do with their molecular structure). On the first view, the same thing may be sweet to one person and sour to another (because it isn't really the thing that is sweet or sour, only the varying sensations). On the third view, what is sweet is sweet is sweet for everyone (because it isn't determined by people's reactions). One difference between the first and the second view is that only on the second view are things sweet when they are not being tasted.