This is a problem that arises when considering psychologism and logic as rival accounts of what
thinking is. Versions of this go back as far as Frege, probably further. Karl Popper also addresses this implicitly when he considers “objective knowledge” versus the psychological fact of thinking. Similarly, Donald Davidson is interested in reasons as causes of physical actions, but his thesis in “Actions, Reasons, and Causes” would presumably apply to mental actions (thoughts) as well. On the other hand, Nietzsche wrote, “It is just an illusion that one thought is the immediate cause of another thought. The events which are actually connected are played out below our consciousness.” (What he has in mind here is of great interest, but outside the scope of my OP.)

There are reasons for questioning whether the concept of causality as such is even a useful one; the excellent OP by @T Clark and subsequent discussion shows why. But for this thread, I’m going to accept the idea that our common understanding and use of “cause” is meaningful, and refers to a genuine phenomenon in the world. As @T Clark allows, “It works for certain everyday events at human scale, e.g. if I push the grocery cart it moves.” I think we should see “thought-to-thought connection” as another example of an everyday event at human scale – at any rate, that’s the premise of what follows.

The question is whether the movement from one thought to another is a type of causation, and if it is not, how should we describe this familiar experience? I’m not going to defend a particular answer to this question, but try to show why the problem needs consideration.

Here’s the set-up:

A. I think: “I wonder how my friend Ann is doing.”

B. I then think: “It’s her birthday soon; I must get her a present.”

The most standard description of what’s going on here is, I believe, something like: “The first thought reminded me of the second thought,” or “When I thought of Ann, I remembered it was her birthday soon, which reminded me that I want to get her a present.”

But can we also speak of this in casual terms? Again, this seems in accord with common usage. We might say, “Thinking of Ann caused me to remember her birthday.” But perhaps this is just loose talk.

What sense of “thought” is being appealed to here? Is it “thought” as a mental event (presumably grounded in brain activity), or is it “thought” as proposition or propositional content?

Google’s ever-helpful chat-program – presumably reflecting some kind of cyberworld consensus – would like to straighten this out for us:

“Causation involves a physical connection between events, while entailment is a relationship between propositions.”

The first thing to notice about Google’s “physical connection between events” idea is that it begs the question of what mental-to-physical causation would have to consist of. The connection between a thought (“I want to call my friend”) and an action (I pick up my phone) is stipulated to be a physical connection, if it is indeed to be causal. Now our helpful but not very deep chat-program has in “mind” things like billiard balls, of course, where both cause and effect are physical. But this won’t do as an allegedly obvious description of mental-to-physical causation; it would need to be demonstrated and argued for, showing that the “mental” part is really somehow physical.

A causal connection between thoughts is even less acceptable as a “physical connection between events.” If it were causal, it would be an example of mental-to-mental causation, with no reference to the physical whatsoever.

Or would it? Putting aside the question of physical reduction, is a thought a strictly mental or psychological event, something that happens in time?

We need two important discriminations here. First: There is an entire debate that could be launched at this point concerning the relation between minds and brains, between thoughts and neural events. That is not the debate I want to open. If you believe, with Google’s chat-program, that any causal connection must be physical, and that therefore, if thoughts cause other thoughts, then they can only do so via a description in terms of neuronal activity – not much in this OP will really interest you. The physicalist reduction of mind to brain is a respectable (though I think misguided) position, but the problem I’m raising assumes it is incorrect.

Second: We need to further distinguish the two senses of “thought.” Let’s return to the two quoted statements in A and B. We can view these statements in two distinct ways. On one view, they represent thoughts that occur to a particular mind at a particular time. They are psychological events. They begin and end. They will presumably be correlated in some way with brain events but, as above, we’re assuming they don’t reduce to brain events.

The other way to view them is as propositions. On that view, neither A nor B is to be identified with any particular “mental utterance.” They are not thoughts, except in Frege’s use of the term, by which he only means what we now call propositions.

Since propositions, on the common understanding, occupy no physical or mental space – this is somewhat mysterious, but it is the usual construal – they can hardly be the subjects or objects of causality, or so it would appear. (In this they are rather like numbers: we certainly don’t say that 2 + 2 cause 4.)

But the first view, where the quoted statements in A and B are my thoughts or your thoughts, can be analyzed differently. We all know the experience of having a thought, A, which leads directly to another thought, B. Ordinary language endorses statements like my “When I thought of Ann (A), it made me remember that I hadn’t yet gotten her birthday present (B).” How strong is “made me remember” in such a phrasing? This can be debated, but the causal implication remains, or at the least a cause-like influence: Thought B would not have occurred to me, had I not had Thought A first, and this happened very directly, much as an efficient cause operates.

To keep this use of “thought” distinct from the propositional content of a thought, I’ll call it a “W2 thought” from now on, in honor of Popper’s World 2 of mental events. (Those of you who know Popper will recognize my set-up as Popperian in origin, with propositions in this case being examples of World 3 objects.) The key distinction between thoughts and propositions, on this view, is that W2 thoughts occur in time and space, as individual mental or psychological utterances, whereas propositions (again, somewhat mysteriously) do not. My W2 thought can never be the same thought as your W2 thought, even if we think the same proposition. That is because it occurs in my mind, not yours.

As philosophers, we’ll probably also want to give a more nuanced story about this, which might include an account of how the mental supervenes on the physical, such that brains and W2 thoughts can thrive together. But even without such a story, there’s a clear sense that the arrow of causation (or influence, if you prefer) goes one way and not the other, when it comes to W2 thoughts – just as it does in the physical world. And we certainly make counter-factual assertions about how W2 thoughts link together (“If I hadn’t W2-thought about Ann, I wouldn’t have W2-thought of her birthday present”).

So the question I want to pose is simply this: When we speak of one thought causing another, are we speaking about W2 thoughts, or about propositions?

If the former, then we need a theory about how psychological events can be causative. I think (though I’m not certain) that such a theory – long the Holy Grail of this area of inquiry – would equally explain both mental and physical effects. In other words, if we can understand how my thought of Ann causes me to think about her birthday present, we can also understand how that thought might cause me to pick up the phone and call her. This seems to be a general problem about the causal power of W2 thoughts, not a specific one about mental-to-mental causation.

If the latter, then we need a theory about how propositions can be causative. Why this is different from W2 thoughts being causative is crucial to understand. If a proposition can be said to have causal power, then this must be so regardless of any particular instantiation or utterance of that proposition. Such a theory would argue that the power is not psychological but rational. The fact that the W2 thought of Ann leads to the W2 thought about her birthday is not (adequately) explained merely by the congruence in time, in my brain, of the two thoughts. Rather, it is explained by the rational connection between the meaning or content of the two thoughts – that is, their propositional content.

This idea is perhaps clearer if we substitute a genuine entailment for the less formal example of Ann and her birthday: I think, “If all humans are mortal and everything that is mortal is beautiful, and Socrates is human, then Socrates is beautiful.” I then think, “Socrates is mortal”. Lastly, I think, “Therefore Socrates is beautiful”. Why do I think “Therefore Socrates is beautiful”? Am I “caused” to do so? Not everyone who has the first two thoughts – understood now as W2 thoughts – would go on to conclude that Socrates is beautiful. Only someone who understands the connections of propositional content will see the necessity of this. So a great deal hangs on whether this kind of necessity – a strictly logical or rational necessity – can ever be considered causative.

This takes us back to the Google chatbot’s confident statement that “causation involves a physical connection between events, while entailment is a relationship between propositions.” We have good reason to doubt the first half of that statement; I’m suggesting that the second half is also suspicious, if it’s meant to imply that entailment is not or cannot be causative. Entailment is indeed “a relationship between propositions,” and the connection between the two Ann thoughts is also “a relationship between propositions,” but this formulation is nearly empty. What do we actually understand about the nature of these relationships?

Not enough, I believe. The floor is open. And if I were critiquing my own OP, I’d start by asking, “Is it really possible to ‛have’ a W2 thought without understanding its propositional content (assuming it has one)?” What do we mean when we talk about “having a thought”, anyway? I’d also raise the question of whether asking “Can a Thought Cause Another Thought?” is clear enough, without first being much more specific about what we want “cause” to cover.