Intentionality is not just mark of the mental processes, it is a mark of all the things of which we think.
Namely, we can’t think about a thing, we can’t remember a thing, see it or imagine it, and so on, without that thing being in same moment thought of, remembered, seen, imagined, and so on.By “thing” here, I mean concrete thing, property, event or a universal (notion, concept). Same goes for both cases.
Take for example case of a thing that we remember. It is inevitably intentional content of the act of remembering. Or if it is a thing we see, inevitably it is intentional content of the act of seeing.
So, not just that all mental acts are intentional, but there are no things we can think about which would not in same time be content of intentional acts.
It seems to me, that If we want to properly understand things then, we need to approach them as a content of intentional acts, and not separately abstracted from the intentional acts. The only way to leave the content as something separated from intentional acts is not to remember/imagine/see it etc…, and not to think of it. But that is hardly acceptable in philosophy.
Connected to this one can criticize the Kantian ‘ding an sich’, a notion mentioned but not thought about – an empty nothing and not even that (one can after all think about ‘nothing’), and also criticize representational theory of mind. What are those representations of? No way to tell except by connecting to (what is taken to be) other representations, we are again left with real world consisting of (or being) things-in-themselves – something which is empty nothing, and not even that. But that didn’t stop neither Kant nor representationalists to assume causal connection between those empty notions and the mind; underlying those representations themselves. Here is how somebody represented (pun intended) this situation :
The serpent bites its own tail. But it is only after a long period of mastication that he recognizes the serpent taste in what he is devouring. So the serpent stops. But after a certain while, finding nothing else to eat, he starts chewing again. Then he comes to the point of having his head in his mouth. That’s what he calls ‘a theory of knowledge. – Cahiers, Paul Valéry