Thanks for the response.
I will return to what I meant by 'complex' in a minute. But nothing in your response undermines my point, which is that similarity between thought and object-of-thought is made impossible by materialism. It is true that "a chain of physical causality [runs] from the outside world to brain states via sense organs", and it is also true that brain states will be different based on the causality imparted by the world, as well as previous brain states, genetics, drugs, etc. But causality is not likeness. A brain-state caused by a tree is not of itself the same as a thought about a tree, any more than something else caused by a tree (like the sound of rustling in the wind) is the same as a thought about a tree. The latter needs likeness, not just causality.
> yet you dismiss the possibility of it being anything physical without offering the slightest justification.
The only way there can be likeness if materialism is true is physical similarity. The brain would have to be physically like a tree when Bob had the thought of a tree for there to be likeness. If we take materialism to mean that everything can be reduced to matter, then there cannot be any likeness that is not of matter. And this means a physical/material/atomic likeness is the only possibility. Yet we know there is no such likeness.
The concept of emergence doesn't overcome this problem. It must either produce a material likeness, which we know doesn't happen, or it must cause some kind of likeness which is real-but-immaterial, which is a problem for materialism.
The 'complexity' I referred to was in the context of how we know there is likeness between thought and reality vs that we know there is said likeness. We can know that the likeness exists without knowing how it exists, or being able to describe it in detail. The point was that no matter how complex the mind/brain/process may be, we know likeness must exist in one way or another, because without likeness there is no knowledge of reality. The complexity of the process, whatever the process is, is irrelevant to the need for likeness. That was my point in raising complexity; I should have been clearer. Complexity does not change the fact that the only likeness for materialism can be a physical likeness.
The CD is not a valid parallel. There is a chain of causality between the laser's interaction with the disc, and the sounds produced by vibrating speakers. A chain of causality is not a likeness, as I stated above. Further, a CD has no knowledge of its contents, so there is no need for likeness (physical or otherwise) between its imprint and the sounds it causes. We have knowledge of the world, so there is need for likeness.
Anyway, the nub of the matter is that you need to show how there can be likeness that is not physical likeness if materialism is true, or you need to show that likeness is not necessary for a thought to be about reality.