It is good to see that we are more-or-less on the same page about what the issue is, but I can also see, from some of the things you say here, that I need to do a more thorough job in explaining myself. In the following, I will give a “Bill and Bob” example showing how I think chain-of-causality answers what I will call, for brevity, the central question (what is it that makes a thought be one about reality?), and I will make more clear how this view stands in relation to similarity.
I realized, reading your reply, that I had not make it clear that chain-of-causality is about information flow: how does information about reality shape our thoughts so that they conform to reality? It also gives an account of how and why this quite often does not work, leading to people thinking and believing falsehoods.
I also realized that its premises can be weakened, and as a consequence, it is not explicitly a materialist hypothesis. Previously, I wrote “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” [i.e. what I called the central question above]. That second (emphasized) premise can be weakened to “changes in mental states are caused by sensory inputs and ongoing mental processes.” This modified premise seems more congenial to anti-materialists, given the considerable empirical evidence that this is in fact what happens.
Without further ado, here’s my Bill-and-Bob scenario. I have altered from your original one to make Bill’s mistake seem more plausible.
Bill and Bob are hiking a mountain trail when Bill sees a stone, having glittering yellow inclusions, beside the path. "We've found gold!" he exclaims, picking it up and showing it to Bob, but Bob takes it, examines it closely, and replies "it's just iron pyrite, Bill." - and he is right.
So what causal chains might lead up to this exchange? There are all sorts of possibilities, going back to the big bang, though it seems likely they will mostly be variations on a theme. We might pick up the story with humans discovering gold and naming it, something that happened independently in many different places by many different people, and the names they gave it were, for the most part, independent, related only by their being all names given by various people to this one substance (I am making an issue of naming here in order to use it in making a point later.)
The causal story continues with gold being widely seen as valuable on account of various properties, such as being nearly inert and having a density and abundance in the sweet spot for being a portable currency. At some point, it is discovered that another substance found in nature looks superficially like gold, but is not, and sometimes can quickly be distinguished from gold on account of its tendency to form characteristic cubic crystals.
Most of what Bob and Bill hold to be truths about gold has probably come to them via language: even though they have probably seen and held gold artifacts, it is likely that they had already been primed for those experiences by being told what gold is like, and it seems highly likely that this is how they came to know that it is valuable, as well as being how Bob came to know about iron pyrite and ways of distinguishing it from gold ore.
Note that having much of this chain being in the form of language is not a problem for either this causal story or for physicalism, as communication of information via language is itself widely and uncontroversially seen to be a causal process.
From this point, it is very easy to see how Bob's causal chain could lead to him making his true statement, while Bill's did not - here, it is just a matter of Bob's chain conveying more complete factual information. In other cases, it might involve things like Bill receiving false information and taking it to be credible, or Bob having a better memory or a greater bias towards skepticism than Bill.
Note that the causal chain of information flow does not stop at one’s sense organs: as postulated in my premises, prior mental states and ongoing or further mental processes are also involved in how the mental state changes in response to the sensory input. For example, if Bob is more skeptical than Bill, it might be a result of painful past experiences biasing him to be wary.
Similarly, if someone receives some unwelcome information, and, as a result of their reaction to it, says something false, this is not something happening outside of the chain of causality. It is also one of at least five mechanisms by which false thoughts can arise - the others I have in mind are incorrect reasoning (including, but not limited to, motivated reasoning and rationalization in the psychological sense), reasoning from incomplete information (whether known to be incomplete or not), simple misunderstanding, and deliberate attempts to deceive (aka lying.)
Now let's move on to what this does and does not imply. In part 1 of your latest post you write "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"),..." The quote-within-the-quote here is taken from this passage: " 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." [Since I wrote this, I weakened the second premise, but that has no consequences for the issue here.]
What this says is that there is a putative answer to the central question that does not make use of the premise that true thoughts are like the external reality that they are about [1].
One of the things this neither says directly nor implies is that this answer is predicated on the assumption that thoughts are not like the external reality that they are about (I hope it is obvious that it does not say this directly, and we can see that it does not imply it by noting that you can add either the premise that true thoughts are like the external reality that they are about, or the premise that they are not, and in either case you get a valid argument, the conjunction of which is equivalent to the argument without either premise.)
In other words, this is a putative answer to the central question that is completely independent of the question of whether there is a similarity between the thought and reality: if there is similarity, that’s fine but inconsequential, and the same goes if no similarity can be found.
I hope this makes it clear that this is not an attempt to show that there is a way for materialism to be consistent with your thesis that similarity is essential for thoughts to be about reality; On the contrary, it is an argument with implications that go against that thesis. Again, this neither implies nor requires that there cannot be any similarity. Personally, I do not suppose that one could think meaningfully about reality without there being a correspondence, at some more-or-less abstract level, between the thoughts and the real world, and for all I know from what you have said about it, that may or may not be what you mean when you say 'similarity' or 'likeness'. Even if it is, that certainly does not mean that I am rationally obliged to accept your premise that it is the one and only reason why thoughts can be about reality. Quite the opposite, in fact: I suppose that it is information flow along chains of causality that both enables this property of thought and leads to thoughts having a correspondence with reality.
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[1] I am assuming that we can agree that this discussion is limited to considering thoughts in the form of propositions about an objective reality which we assume to exist.
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