Anyway, to the main point. You wrote:
> The point in my penultimate paragraph is this: If we suppose that there is a causal flow from the real world to brain states via the sense organs, and adopt the premise that (changes in) mental states are caused by (changes in) brain states, then we have at least the outlines of an answer to your tacit initial question (what is it that makes a thought being one about reality?) that does not invoke or depend on any concept of likeness or similarity. In what I quoted above, you certainly seem to be saying that the only thing that could make a thought about reality is that it has a similarity (in some nebulous sense, at least) to reality, but here we have another way for a thought to be about reality that does not depend on likeness in any sense.
This clarifies our disagreement well. I will divide my response into two sections: 1. Why it is necessary for a thought to be like reality if a thought is to be about reality, 2. Why a causal chain cannot provide either likeness or a substitute for likeness.
1.
A thought can be right or wrong, but right-ness implies likeness to reality, while wrong-ness implies un-likeness. To illustrate: suppose Bob has a thought about a tree. Let's say it's "this oak tree is made of wood". Now suppose his friend Bill thinks "this oak tree is made of copper". I take it we can premise that Bob is right and Bill is wrong.
But for our premise to be true, their thoughts must be compared to the tree itself. There is no other way to judge their thoughts' right-ness or wrong-ness. For Bob's thought to be right, it must be like the tree (and the opposite for Bill's). Any conception of thought that lacks likeness makes this impossible. If we deny that both Bob's and Bill's thoughts are like the tree (which is necessary if we "do not invoke or depend on any concept of likeness or similarity"), neither Bob nor bill can be right. The concept of right-ness is predicated on that of like-ness. If, therefore, you drop the concept of like-ness, you lose the possibility of right-ness. And in this situation, all thought becomes meaningless, including the thoughts we're having right now about the nature of the mind.
Therefore we can conclude that likeness is necessary for correctness, and therefore for thought to be about reality.
Be aware that my claim is very limited. I am not, right now, making any claim as to what this likeness may be like. I am simply saying that likeness in some sense of the word is necessary for rational thought. Hence I disagree that may claim is 'nebulous'; rather, it's limited.
2.
A causal chain does not rescue your denial of likeness from its implications. As I stated above, causal chains happen everywhere in nature ("the tree causes a rustling sound"). A brain state is one result of a particular causal chain, but there is nothing special about it on that account. There is nothing that gives it the possibility of being rational, or correct. More specifically, a causal chain cannot in itself be 'right' or 'wrong'. It is possible that a causal chain will cause Bob to think a tree is made of wood, and cause Bill to think it's made of copper. This is not enough for Bob to be right and Bill wrong, because the chain has no connection to correct-ness. It can't give us reason for saying "Bob's thoughts are like reality, and Bill's are not". A causal chain is not right or wrong, or rational in any sense. That someone has been caused to think something doesn't mean he has a reason for thinking something. So a causal chain cannot provide a substitute for likeness if we want to rescue the concept of rational thought. Whether or not a brain state causes a mental state is irrelevant to this fact, contrary to what you claim.
Nor can a chain provide physical likeness itself. The materialist view of the mind closes off any other way in which likeness can be achieved, because (if it's consistent) it postulates that brain states, ultimately reducible to fundamental particles, are all that objectively exist. You claim "an understanding of causality may potentially lead us to being able to say... in what way thoughts are similar to reality", but you offer no details. I deny this is so -- causality can't rescue your position; it can't make there be some like-ness that is somehow both material and yet does not involve arrangements of atoms that are similar, which we've already ruled out. I'm happy to hear details if you think otherwise.
To summarize:
1. Rational thought requires (the possibility of) likeness 2. Materialism makes likeness impossible, because it reduces the mind to physical states, and a brain's physical state is not like the object of its thought [your arguments about causality do not gainsay this] 3. Therefore materialism makes rational thought impossible, and is therefore false.
So my claim way upthread, that 'there is an immaterial component to our thought, and therefore to our existence' must stand.