Franz Brentano, for whom intentionality is the mark of the mental, is committed to the thesis that all instances of (intrinsic) intentionality are instances of mentality. Propositions and dispositions are apparent counterexamples. For they are nonmental yet intrinsically object-directed. Whether they are also real counterexamples is something we should discuss. This post discusses (Fregean) propositions. Later, dispositions — if I am so disposed.
On one approach, propositions are abstract objects. Since abstracta are categorially barred from being mental, it is clear that if intrinsic intentionality is ascribed to abstract propositions, then the thesis that all instances of intentionality are instances of mentality must be rejected. For specificity, we consider Frege's theory of propositions. He called them Gedanken, thoughts, which is a strangely pyschologistic terminological choice for so anti-psychologistic a logician, but so be it.
A proposition is the sense (Sinn) of a certain sort of sentence in the indicative mood, namely, an indicative sentence from which all indexical elements, if any, such as the tenses of verbs, have been extruded. Consider the following sentence-tokens each of which features a tenseless copula:
1. The sea is blue
2. The sea is blue
3. Die See ist blau
4. Deniz mavidir.
(Since Turkish is an agglutinative language, the copula in the Turkish sentence is the suffix 'dir.')
The (1)-(4) array depicts four sentence-tokens of three sentence-types expressing exactly one proposition. Intuitively, the four sentences say the same thing, or to be precise, can be used by people to say the same thing. That same thing is the proposition they express, or to be precise, that people express by uttering them. The proposition is one to their many. And unlike the sentence-tokens, it is nonphysical, which has the epistemological consequence that it, unlike the sentence-tokens, cannot be seen with the eyes. It is 'seen' (understood) with the mind. Frege is a sort of latter-day Platonist.
So one reason to introduce propositions is to account for the fact that the same meaning-content can be expressed by different people using different sentences of different languages. Another reason to posit propositions is to have a stable entity to serve as vehicle of the truth-values. The idea is that it is the proposition that is primarily either true or false. Given that a proposition is true, then any sentence expressing it is derivatively true.
There is quite a lot to be said for the view that a sentence-token cannot be a primary truth-bearer. For how could a string of marks on paper, or pixels on a screen, be either true or false? Nothing can be either true or false unless it has meaning, but how could mere physical marks (intrinsically) mean anything? Merely physical marks, as such, are meaningless. You can't get blood from a stone, or meaning from meat, no matter how hard you squeeze, and no matter how wondrously organized the meat.
Fregean propositions are especially useful when it comes to the necessary truths expressed by such sentences as '7 is prime.' A necessary truth is true in all possible worlds, including those worlds in which there is nothing physical and so no means of physically expressing truths. If truth is taken to be a property of physical items or any contingent item, then it might be difficult to account for the existence of necessary truths. The Fregean can handle this problem by saying that propositions, as abstract objects, exist in all possible worlds, and that true ones have the property of being true in all possible worlds. The Fregean can also explain how there can be necessary truths in worlds in which there is nothing physical and nothing mental either.
Propositions also function as the accusatives of the so-called 'propositional attitudes' such as belief. To believe is to believe something. One way to construe this is de dicto: to believe is to stand in a relation to a proposition. Thus if I believe that the river Charles is polluted, then the intentional object of the belief is the proposition expressed by 'The river Charles is polluted.' (Of course, there is also a de re way of construing the belief in question: To believe that the Charles is polluted is to believe, of the river Charles, that is is polluted.)
Well, suppose one endorses a theory of propositions such as the one just sketched. You have these necessarily existent Platonic entities called propositions some of which are true and some of which are false. My believing that p is an intentional state directed upon p; but is it not also the case that p is directed upon the world, or upon a truth-making state of affairs in the world in the case in which p is true?
But now it looks as if we have two sorts of intentionality, call them noetic and noematic, to borrow some terminology from Husserl. Noetic intentionality connects a mental state (in Frege's Second Reich) to a proposition (in Frege's Third Reich), and noematic intentionality connects, or purports to connect, a proposition to an object in Frege's First Reich. Frege wouldn't think of this object as a state of affairs or concrete fact, of course, but we might. (The peculiarities of Frege's actual views don't matter for this discussion.)
The problem for Brentano's thesis above is that propositions — which are abstract objects — seem to display intrinsic aboutness: they are about the concrete world or states of affairs in the world. Thus the proposition expressed by 'The Charles is polluted' is intrinsically about either the river Charles or else about the state of affairs, The Charles River's being polluted. Intrinsically, because the proposition's being about what it is about does not depend on anyone's interpretation.
If this is right, then some instances of intentionality are not only not conscious but not possibly conscious. Does this refute Brentano's thesis? Brentano himself denied that there were such irrealia as propositions and so he would not take propositions as posing any threat to his thesis. But if there are (Fregean) propositions, then I think they would count as counterexamples to Brentano's thesis about intentionality.
Is there a way to uphold Brentano's thesis that only the mental is intrinsically intentional? Yes, if there is a way to identify propositions with thoughts or rather content-laden thinkings. My thinking that 7 is prime is intrinsically intentional. Unfortunately, my thinking is contingent whereas the content of my thinking is necessarily true and hence necessarily existent. To identify propositions with content-laden thinkings one would have to take the thinkings to inhere in a necessarily existent mind such as the mind of God.
So I end on an aporetic note. Intentionality cannot be the mark of the mental if there are Fregean propositions. But given that there are necessary truths and that truth-bearers cannot be physical items, then only way to avoid Fregean propositions is by identifying propositions with divine thoughts, in which case they are Gedanken after all.