In my first post yesterday, I asked us to accept the following definition, for the sake of argument.
(1) Intentionality: the existence of some thoughts depends on the existence of external objects
Note my comment 'for the sake of argument'. As Bill Vallicella has commented on this post here, this definition does not coincide with Franz Brentano's definition. But then Brentano's use of the term does not coincide with the medieval usage, which is different again. The thesis I am trying to capture here is what Stephen Neale has called the 'object-dependence' of certain thoughts: that certain thoughts are intrinsically 'about' an object. Note also that by 'external' I mean genuinely separable and distinct from the person thinking: such that it could exist (in some sense) without the person thinking about it, but also such that if it did not exist, the thought could not exist.
I am exploring the reasons that some philosophers have given for believing the thesis of intentionality or 'object dependence' as I am characterising here. The first step is to characterise thought in terms of the language expressing it, in a way that implies (1) **, as follows.
(2A) A thought is the same as the signification of the proposition* expressing that thought
(2B) The signification of some propositions (namely singular propositions) depends on the existence of external objects
This doesn't get us that far, but it focuses the discussion. It takes us from 'thought', which in a wide sense could include any mental phenomenon such as an emotion, a musical phrase, an idea or mental image, to something narrower whose structure and composition is closely related to the structure and composition of language. This move was crucial in the development of modern analytic philosophy (in the hands of Russell, Wittgenstein and others). It was also taken for granted by the late 13th century modist philosophers who held that thought is presupposed by signification (significare praesupponit intelligere).
*I am using the term 'proposition' in its traditional sense, i.e. as signifying an indicative sentence, rather than the meaning of an indicative sentence.
** Note that, in this and all subsequent posts, I will be giving premisses that support the contentious premiss of the previous post, in terms of a (to me) uncontentious premiss labelled 'A', and a contentious premiss labelled 'B'.