By thinking, I mean something more than the capacity to calculate or process information. What I mean by thinking is something like having a thought about something. I guess the best place to start thinking about thinking is me. I think I am a thinking thing. As such, I think about stuff. I believe, hope, entertain, consider, fear, and wish all the time. That’s what I mean by thinking. And a machine cannot do it, not now and not ever.
By a machine I mean a material object composed of physical parts arranged in such a way as they perform a certain set of functions. Old fashioned clocks are made of all sorts of physical parts that when put to work together make some hands go round at a regular pace thus enabling us to say, “Ah, it’s 3 o’clock. Time for tea.” This is true of all machines, even very complicated digital machines.
There are two simple reasons why anything so described won’t be able to have thoughts of the kind I can have. First, any physical set of parts composing a machine is itself made of physical parts. Perhaps there is a smallest possible part, what the ancients referred to as an atom. Consider such a thing. It is a physical object, a very very small bit of the universe. But it lacks anything like thought. An atom (or proton, quark, or whatever) can’t believe, hope, consider, or any other mental activity. But if a machine (a computer, for example) is composed of physical parts which in turn are composed of small parts none of which have any chance of thinking, then the larger thing, the machine, hasn’t any chance of thinking either.
The second reason is this: Consider any arrangement of physical parts. Now consider a belief. A belief has on the one hand an attitude – ‘I believe that…”, and on the other hand it has content, what I believe. Now, think of the arrangement of the physical parts. How does such an arrangement have both an attitude and the content toward which the attitude is directed? Upon a moment’s reflection, it is clear that there just isn’t anything about any arrangement of any set of physical parts that could be an attitude of belief with a content toward which the attitude is directed. Wandering around the machine, one just won’t be able to say, ‘look! there is the content and over there is the attitude.’
“Hold on old boy, surely sentences have content and sentences are physical things. Ergo, physical things can have content.” Yes, but sentences don’t have their own content. I use a sentence to, say, assert a belief. Another person, upon reading it, grasps the content of the sentence, considers its truth-value and then either believes it or doesn’t. None of these properties are properties of the sentence alone. Rather, I–a thinking thing–use the sentence to communicate to another thinking thing. The sentence gets its content from thinking things, it doesn’t have it without them.
If physical things can’t think, then it is both true that thinking things cannot be physical and that physical things cannot be thinking things. So, I am not my body and no machine can think. Both substance dualism, the view that I am an immaterial thing, and the other view that I don’t have a name for (perhaps, ‘strong non-artificial intelligence’) are less popular now than they used to be. However, popularity does not entail truth. And, since no physical thing can think, they are very likely true.
All the above can be found in a vastly superior piece of philosophy by Alvin Plantinga called “Against Materialism” Faith and Philosophy 23 (2006): 3-32. And, as Plantinga says, it can be found in the works of Leibniz.