Section VII of Book I of David Hume's A Treatise of Human Nature is relevant to recent investigations of ours into belief, existence, assertion, and the unity of the proposition. In this section of the Treatise, Hume anticipates Kant's thesis that 'exists' is not a real predicate, and Brentano's claim that the essence of judgment cannot consist in the combining of distinct concepts.
1. For Hume, a belief is an idea to which we assent. Hume is thinking of occurrent beliefs, not latent or dispositional beliefs. To believe that God exists, for example, is to assent to the idea (concept) of God. Thus as I read Hume, believing is not a propositional attitude in Russell's sense: the believer in believing is not related to a proposition but to a non-propositional object, an idea. In every occurrent belief one must distinguish the act of believing from the content believed; but it is not obvious that this content must be a proposition.
In the case of singular existential beliefs or judgments (that God exists, that Vulcan does not exist, etc.) it is very tempting to say that the belief-content is a non-propositional object. For if the object of the existential belief were propositional, then it would have to have the subject-predicate logical form a exists when it is not at all clear that 'exists' is a first-level predicate or existence a first-level property. As Hume points out, employing one of his signature 'riffs,' "The idea of existence is nothing different from the idea of any object. . . ." (Selby-Bigge, 94) If I conceive of God as existent, I "in reality make no addition to or alteration on our first idea."
To conceive of X and to conceive of X as existent are the same: existence makes no addition to X's concept. This is an important point and seems wholly correct, and it doesn't matter what 'X' denotes. Consider the unique X so defined that X's existence is included in its concept. There is no incoherence in maintaining that this object does not exist. Or compare the golden mountain and the existent golden mountain. Neither exist. 'The existent golden mountain does not exist' is not a contradiction in the way 'The snow-covered golden mountain is not snow-covered' is a contradiction. And yet there is a huge difference between a golden mountain's existing and its not existing. One should conclude from this, not that there is 'no such thing' as existence, but that existence is not part of anything's concept. If you were to tell me that existence is part of the concept existent golden mountain, then I would reply that the existence that is includable in concepts is not the genuine article; it is not the existence we mean when we say that such-and-such exists. Nothing seems clearer than that existence, the genuine article! (stamp the foot, pound the table, release the safety) is not includable in anything's concept. Surely Hume and Kant and Brentano and Aquinas and Santayana and a host of others are right about this.
But if so, if, to put the point in an alternative way, existence is not a first-level property, then there is no atomic proposition God exists or atomic state of affairs God's existence, and to believe that God exists is not to stand in the belief relation to a proposition.
2. So what are we doing when we make existential judgments?
(A) When I think of God, think of him as existent, or believe him to be existent, "my idea of him neither encreases [sic] nor diminishes." (94) But (B) "there is a great difference betwixt the simple conception of the existence of an object, and the belief of it. . . ." And because (C) ". . . this difference lies not in the parts or the composition of the idea, which we conceive . . ." it follows that (D) ". . . it must lie in the manner, in which we conceive it." (Selby-Bigge 94-95)
To believe that Peter exists is not to believe that Peter has a certain property, the property of existence: there is no such property. With a sidelong glance in the direction of Brentano we might say that to believe that Peter exists is to accept Peter, and to believe that he does not exist is to reject Peter. Positive belief (acceptance) and negative belief (rejection) are mental acts external to the belief-content. The belief-content is non-propositional. Brentano thinks that this is the case for all contents including those that are not obviously existential. For Brentano, to believe that Peter is sick is to accept sick-Peter, and to believe that Peter is not sich is to reject sick-Peter. The object or accusative of the act of believing is structured, but not propositionally: the accusative is sick-Peter, not Peter is sick.
This is not quite what Hume means, but I would say that Hume in on the way to the Brentano view in that Hume holds that the contents of belief (in some cases at any rate) are non-propositional. Unlike Brentano, Hume does not see judging as a category of act distinct from conceiving, but as a conceiving with increased "force and vivacity":
So that as belief does nothing but vary the manner, in which we conceive any object, it can only bestow on our ideas an additional force and vivacity. An opinion, therefore, or a belief may be most accurately defin'd, A LIVELY IDEA RELATED TO OR ASSOCIATED WITH A PRESENT IMPRESSION." (Selby-Bigge, p. 96, emphasis in original)
There is much that is murky here. For example, how can a belief in God be a lively idea related to an impression when there is no impression of God?