It seems to me that there are two common usages of the term. One holds that thinking is everything that the conscious mind does. That would include perception, mental arithmetic, remembering a phone number, or conjuring up an image of a pink unicorn. On this definition, thinking simply equates to conscious cognitive processes. I think this definition is too broad, and we make more scientific and philosophical progress if we tighten it up a bit. 

People of a psychodynamic persuasion might even want to talk about 'unconscious thinking', but I think that makes the term so broad as to be quite useless. Of course there are tremendously important unconscious cognitive processes shaping the way we make sense of the world, but 'thinking' seems to me to be quintessentially conscious.

Let's take Rodin's The Thinker as an example. Here's someone who is more than just conscious. He is struggling with a problem, cognitively trying to get from A to B. One important point about thinking is that it is active; it is something that we do¹. That's why I, following Vygotsky ², prefer the active term 'thinking' to the passive term 'thought'. If you like, we can call 'thought' the product of the active process of 'thinking.' But I think that sticking to the active term keeps the special qualities of thinking at the forefront, and that's important for what we have to do. 

What about the experience of thinking; what it seems like from the inside? If we could put ourselves in Rodin's character's head, what would we see, hear, and feel? Phenomenology can be misleading here, as it can be throughout psychology. Just because an experience seems to be a certain way, that doesn't mean it's a true guide to what's going on our brains. But it's hard to deny an experience which has, I believe, such a strong subjective character. We know what it is like to be thinkers, and we can usefully talk about it. 

I like to use a Hollywood movie as an example here. In the 2000 romcom What Women Want, the character played by Mel Gibson has a freak accident which leads him to be able to read women's minds. I'd ask you to excuse the dodgy gender politics on show here, and focus instead on how the movie-makers portray the process of thinking. When Gibson tunes into a woman's thought processes, he hears language. He doesn't see pictures or abstract symbols. He hears a voice, one that is supposed to be private, linking together the unsuspecting victim's experience into a verbal stream of consciousness. In comic books, thought bubbles are usually full of words, not pictures. I would hold that this view of thinking is the one that makes the most sense to us, because it is the one that most closely matches our experience.

In the Radiolab show, I suggest that we can get a handle on what thinking is by introspecting on this experience. If we try to imagine an episode when we are doing something that we would intuitively describe as thinking (say, while we're walking to work or soaking in the bathtub), we have a sense of a flow of inner speech. Our thinking has a verbal quality. We feel as though we are talking to ourselves: not all the time, perhaps, but for an important part of it. (Once again, though, phenomenology might be misleading. It might seem that we have words in our heads when we don't actually. And sometimes, as I'll try to explain, we might have words in our heads which, for the moment, don't sound like words.)

So my narrower definition of 'thinking' goes like this. Thinking is conscious and it is active. It is the kind of cognitive process that can make new connections and create meaning. It is dialogic: it has the quality of an internal conversation between different perspectives, although the 'give-and-take' quality of external dialogues may not always be immediately obvious. And it is linguistic: verbal for those of us who use spoken language, visual for those of us who use sign language to communicate with others and with ourselves. 

So far, so circular. I am claiming that language is necessary for thinking, and then I'm claiming that thinking is defined in terms of its reliance on language. That won't do. But now that we have a slightly clearer sense of what thinking is, we can try to define it in terms of other things that are going on, cognitively and perhaps neurologically. And then we can perhaps make some progress. 

I have said that thinking is inner speech. That's a strong claim, and it requires another step to the argument. We usually assume that inner speech is just one homogeneous kind of thing: a flow of words in the head which appear to us, subjectively, like heard language. I think we need to move beyond this view. I would say instead that there are (at least) two kinds of inner speech, what I have called condensed and expanded ³. Their existence is implied in Vygotsky's writings, but he never spelled it out quite like this. Understanding why these two kinds of inner speech exist requires us to think about where inner speech comes from: how it develops, and particularly how it is transformed as it changes from external to internal speech.

Vygotsky² proposed that this process of transformation involves both semantic and syntactic changes. In a nutshell, the language that is to be internalized becomes abbreviated, so that inner speech becomes a 'note-form' version of the external dialogue from which it derives. In its condensed form, the language that forms inner speech has all of its acoustic properties stripped away, losing the qualities of tone, accent, timbre and pitch that distinguish spoken language. Vygotsky referred to this stage of ultra-abbreviated internal language as 'thinking in pure meanings' ². We see some aspects of this process in action in children's private speech, which can be seen to undergo the same transformational processes as it gradually becomes internalized. 

It is this category of abbreviated inner speech that I have called condensed inner speech. In this kind of thinking, we are still using language, but it may not subjectively seem like spoken language (because the acoustic properties of language have been stripped away). At other times, our thinking takes the form of a second kind of inner speech, expanded inner speech, where subjectively we do experience a full-blown internal dialogue playing out in our minds. We have a sense of participating in a true internal conversation, with one point of view answering another, just like a dialogue spoken aloud between two people.