Language lets us treat our thoughts as though they were much like ordinary things. Suppose you meet someone who is trying to solve a problem. You ask what's happening. I'm thinking, you are told. I can see that, you say, but what are you thinking about? Well, I was looking for a way to solve this problem, and I think I've just found one. We speak as though ideas resemble building-blocks that one can find and grasp!
Why do we thing-ify our thoughts? One reason is that this enables us to reapply the wonderful machines our brains contain for understanding worldly things. Another thing it does is help us organize our expeditions in the mental world, much as we find our ways through space. Consider how the strategies we use to find ideas resemble the strategies we use for finding real things: Look in the places they used to be or where they're usually found — but don't keep looking again and again in the same place. Indeed, for many centuries our memory-training arts have been dominated by two techniques. One is based on similarities of sounds, exploiting the capacities of our language-agencies to make connections between words. The other method is based on imagining the items we want to remember as placed in some familiar space, such as a road or room one knows particularly well. This way, we can apply our thing-location skills to keeping track of our ideas.
Our ability to treat ideas as though they were objects goes together with our abilities to reuse our brain-machinery over and over again. Whenever an agency becomes overburdened by a large and complicated structure, we may be able to treat that structure as a simple, single unit by thing-ifying — or, as we usually say, conceptualizing — it. Then, once we replace a larger structure by representing it with a compact symbol-sign, that overloaded agency may be able to continue its work. This way, we can build grand structures of ideas — much as we can build great towers from smaller parts.
I suspect that, as they're represented in the mind, there's little difference between a physical object and an idea. Worldly things are useful to us because they are substantial — that is, because their properties are relatively permanent. Now we don't usually think of ideas as substantial, because they don't have the usual properties of worldly things — such as color, shape, and weight. Yet good ideas must also have substantiality, albeit of a different sort:
No conception or idea could have much use unless it could remain unchanged — and stay in some kind of mental place — for long enough for us to find it when we need it. Nor could we ever achieve a goal unless it could persist for long enough. In short, no mind can work without some stable states or memories.
This may sound as though I'm speaking metaphorically, since a mental place is not exactly like a worldly place. But then, when you think of a place you know, that thought itself is not a worldly place, but only a linkage of memories and processes inside your mind. This wonderful capacity — to think about thoughts as though they were things — is also what enables us to contemplate the products of our thoughts. Without that ability to reflect, we would have no general intelligence — however large our repertoire of special-purpose skills might grow. Of course this same capacity enables us to think such empty thoughts as This statement is about itself, which is true but useless, or This statement is not about itself, which is false and useless, or This statement is false, which is downright paradoxical. Yet the benefit of being able to conceptualize is surely worth the risk that we may sometimes be nonsensical.