An earlier post explains the distinction between mental acts and mental actions. But a logically prior question is whether there are any mental acts in the first place. Suppose I hear the characteristic rumble of a Harley-Davidson engine and then suddenly think of Peter. One cannot move straightaway from such a commonplace observation recorded in ordinary English to talk of mental acts of perceiving and of remembering. This is because 'mental act' is a terminus technicus embedded within a theory. It is a term that drags behind it a load of theoretical baggage that one may not want to take on board. Every mental act is a mental state, a state of a mind. (A state is necessarily a state of something; a mental state is necessarily a state of a mental something.) So talk of mental acts seems to commit one to talk of minds or mental subjects. But their existence is denied by those (Sartre, Butchvarov, et al.) who maintain that consciousness is subjectless. That theoretical denial, however, is consistent with the commonplace that we sometimes hear and remember. On the other hand, talk of mental acts commits one to an act-object distinction, a distinction that adverbialists deny. So although it is obvious that we sometimes hear and remember, it is not obvious that there are mental acts. So we need an argument. Here is one. It is my reconstruction of what I think Laird Addis is saying on p. 71 et passim of Natural Signs: A Theory of Intentionality (Temple University Press, 1989).
1. Consider two states of affairs, S1 and S2. In S1 I am imagining a unicorn (and nothing else) at time t, while in S2 I am imagining a mermaid (and nothing else) at t. S1 and S2 are individually possible, though not jointly compossible.
2. S1 and S2 are numerically different, and this difference requires a ground, a 'difference-maker.'
3. One cannot locate the difference-maker on the side of the object, because there are no unicorns and there are no mermaids. (For an analogy, compare two mathematical sets, one whose sole member is a unicorn, the other whose sole member is a mermaid. These sets are the same set, the null set, inasmuch as there is nothing that could ground their difference.)
4. Since both S1 and S2 involve the same type of mental directedness, namely, imagination, the difference between S1 and S2 cannot be ascribed to a difference in type of mental directedness.
5. Since one and the same subject is the imaginer in both cases, the difference between S1 and S2 is not on the side of the subject. Therefore:
6. There must be something that grounds the difference between S1 and S2, and this all men call 'mental act.'
Cuteness and quinque viae parody aside, there must be something that grounds the difference between S1 and S2 assuming the Difference-Maker Principle: No difference without a difference-maker. This principle strikes me as well-nigh self-evident: how on Earth (or on Twin Earth for that matter) could two different complexes just differ? S1 and S2 are complexes not simples: their numerical difference requires an ontological ground. Suppose someone insisted that the unordered set {Bill, Peter} is just different -- barely different -- from the unordered set {Peter, Bill}. You would show him the door, right? I can swallow a bare difference of simples but not of complexes.
The difference between S1 and S2, then, traces back to a difference between two mental acts. If you ask me what makes these two mental acts different, my answer will be that they differ in their object-directedness: one has unicorn-directedness, the other mermaid-directedness. Perhaps this could be explained further by saying that a mental act is a mental state, where a mental state is a mind's exemplification of an intentional property. So in S1 my mind exemplifies the intentional property unicorn-directedness while in S2 my mind exemplifies the intentional property mermaid-directedness. These property-exemplifications just are the mental acts.
This is pretty close to a Bergmann-Addis assay of the act. If it could be made to work in all details, then we could avoid Meinongianism, Adverbialism, and Sartreanism (Sartvarovianism?). But being an aporetician, I am not sanguine.
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