We infer that they see things differently on the basis of observation and analysis of their different sensory setups. We can infer that they see different ranges of colour, or even only in black and white for example.

It's true that we can get the same or similar information from different sensory modalities, but the sensations themselves are different. All of that information falls inot the category of 'how things appear or present themselves to us'. — Janus

Then what is missing exactly if we know the way they see the world?

It seems natural to think that there must be more to things than just how they appear or seem to be. Of course we can never know more than that, but the fact that we are compelled to think of the 'in itself' has many ramifications for human life. Not in terms of something we know, but in terms of what we can never know. The knowledge here is just self-knowledge. — Janus

But I asked what a "thing in itself" even means. It sounds like a misuse of language. Does it mean to BE the thing in itself? If so, is there a BEING to a chair, table, house, car, or rock? If not then there is nothing missing.

As to our experience of mind I think this is a real minefield. If mind consists only in our experience and judgements would it follow that we know all there is to know about it? Psychedelics and altered states in general show that we have the potential for very different experiences, so it would seem presumptuous to imagine that we have explored all there is to know about what it is possible to experience. — Janus

So the thing in itself includes all states of the thing in the past, present or future? We don't see an apple on the table in the future. We see it in the present. We are talking about the thing in itself at this moment. I'm talking about the right here and now. Do you have direct access to your mind in it's current state? Are you experiencing your mind as "the thing in itself" at this moment?

Even with that said, we can make predictions and get at past causes by our present observations. If we get at the past causes and make accurate predictions, again - what is missing?

Then what is missing exactly if we know the way they see the world? — Harry Hindu

We infer the way they see the world. It doesn't follow that we can see the world that way.

But I asked what a "thing in itself" even means. It sounds like a misuse of language. Does it mean to BE the thing in itself? If so, is there a BEING to a chair, table, house, car, or rock? If not then there is nothing missing. — Harry Hindu

We don't know what things are apart from how they appear to us. Once that is realized it is possible to make the logical distinction between how things are for us, how they are for other animals and how they are in themselves. Some believe that physics shows us how things are in themselves, but the problem is there is no way to know if that is true, and anyway even the quantum physicists say that they can form no coherent picture of the quantum world and that understanding it is only possible via mathematics. What QM does seem to show is that things are not what they seem.

You just quoted the OP out of context, — Bob Ross

No, I didn't do that either.

The biggest problem for indirect realists (that's the title of your OP) is their own assumption that we never experience objects and states of affairs directly. How is knowledge possible even under such conditions? Hence the complexity of Kant's investigation, and its seemingly paradoxical use of two worlds or perspectives. Such problems don't even arise for direct realists or idealists.

Kant begins with the presupposition that our experience is representational and proceeds to correctly conclude that knowledge of the things-in-themselves is thusly impossible. — Bob Ross

Suppose I have a visual experience of shapes and colours. There is no doubt in my mind that this visual experience has been caused by something external to the visual experience itself. There is no doubt in my mind that this visual experience didn't cause itself.

It seems part of the a priori structure of the brain to expect that everything that happens has a cause. This cause may be called the thing-in-itself.

The next question is about the relation between my visual experience of a shape, for example, and the external cause of my experiencing this particular shape.

Either, the cause is identical to the effect, Direct Realism, or the cause is different to the effect, a representation, Indirect Realism.

But we only know the effect and can only reason about the cause.

For the Direct Realist, what information is there in an effect to be able to know its cause?

If it were possible to determine from an effect its cause, it would be possible to look at a broken window and know what caused it to break.

I don't quite get the problem. Let's say that there's something hiding under my bed cover. I cannot see under my cover to see what it is; I can only see the bump in the cover (and maybe the cover moving). Any knowledge I have of the thing under my cover is at best an inference.

It's the same principle with things like Kant's transcendental idealism or indirect realism.

And then on the further extreme the idealist claims that there isn't anything under my cover; there's only the cover, which happens to have a bump (and maybe is moving).