Commenter John put the following question to me:
Which Platonist theories of propositions did you have in mind in your original post, and what are the problems involved in accepting such views?
I had in mind a roughly Fregean theory. One problem with such a view is that it seems to require that propositions possess intrinsic intentionality. Let me explain.
Propositions: A Broadly Fregean Theory Briefly Sketched
On one approach, propositions are abstract items. I am not suggesting that propositions are products of abstraction. I am using 'abstract' in the (misconceived) Quinean way to cover items that are not in space, or in time, and are not causally active or passive. We should add that no mind is an abstract item. Abstracta, then, are neither bodies nor minds. They comprise a third category of entity. Besides propositions, numbers and (mathematical) sets are often given as candidate members of this category. But our topic is propositions.
For specificity, we consider Frege's theory of propositions. He called them Gedanken, thoughts, which is a strangely psychologistic terminological choice for so anti-psychologistic a logician, but so be it. Like its German counterpart, the English 'thought' is ambiguous. It could refer to an act of thinking, a mental act, or it it could refer to the intentional object or accusative of such an act. Some use the word 'content,' but it has the disadvantage of suggesting something contained in the act of thinking. But when I think of the river Charles, said river is not literally contained in my act of thinking. A fortiori for Boston's Scollay Square which I am now thinking about: it no longer exists and so cannot be contained in anything. The same is true when I think that the Charles is polluted or that Scollay Square was a magnet for sailors on shore leave. Those propositions are not psychological realities really contained in my or anyone's acts of thinking. And of course they are not literally in the head. You could say that they are in the mind, but only if you mean that they are before the mind.
A proposition for Frege 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 (assertively) uttering them or otherwise encoding them. The proposition is one to their many. (I have just sounded a Platonic theme.) And unlike the sentence-tokens, the proposition is nonphysical, which has the epistemological consequence that it, unlike the sentence-tokens, cannot be seen with the eyes or heard with the ears. It is 'seen' (understood) with the mind. Herewith, a second Platonic theme. 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 or sense can be expressed by different people using different sentences of different languages. We also need to account for the fact that the same thought can be expressed by the same person at different times in the same or different languages. Another reason to posit propositions is to have a stable entity to serve as vehicle of the truth-values. 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. Similarly with judgments and beliefs: they are derivatively true if true. For Frege, propositions are the primary truth bearers or vehicles of the truth-values.
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. Therefore, a string of marks cannot be either true or false. It is the office of minds to mean. Matter means nothing.
One could agree that a string of marks or a sequence of noises cannot, as such, attract a truth-value, but balk at the inference that therefore propositional meanings (senses) are self-subsistent, mind-independent abstract items. One might plump for what could be called an 'Aristotelian' theory of propositions according to which a sentence has all the meaning it needs to attract a truth-value in virtue of its being thoughtfully uttered or otherwise tokened by someone with the intention of making a claim about the world. The propositional sense would then be a one-IN-many and not a Platonic one-OVER-many. The propositional sense would be a unitary sense but not a sense that could exist on its own apart from minds or mean anything apart from minds.
But how would the Aristotelian account for necessary truths, including the truths of logic, which are true in worlds in which there are no minds? Here the Platonist has an opportunity for rejoinder. 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 are no minds and/or 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 necessarily 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 cannot just believe. One way to construe this is de dicto: to believe is to stand in a relation to a proposition or dictum. Thus if I believe that the river Charles is polluted, then the intentional object of the occurrent belief state 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.)
A Consideration Contra
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. There are all of these entities that there could have been. Each necessarily exists although only some are necessarily true. As necessarily existent and indeed necessarily existent in themselves and from themselves, they have no need of minds to 'support' them. Hence they are not mere accusatives of mental acts. They are apt to become accusatives but they are not essentially accusatives. They can exist without being accusatives of any mind. To borrow a phrase from Bernard Bolzano, they are Saetze an sich. They are made for the mind, and transparent to mind, but they don't depend for their existence on any mind, finite or infinite.
Even more salient for present purposes is that these Platonic propositions are not only existent in themselves but also meaningful in themselves: they do not derive their meaning from minds. It follows that they possess intrinsic intentionality. At this juncture an aporetic tetrad obtrudes itself.
A. Fregean propositions are non-mental representations: they are intrinsically representative of state of affairs in the world.
B. Fregean propositions are abstract items.
C. No abstract item possesses intrinsic representational power.
D. Fregean propositions exist.
The limbs of the tetrad cannot all be true. One can therefore reasonably argue from the conjunction of the first three to the negation of the fourth.
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