In the natural sciences, it doesn’t matter how you phrase your theories, as long as you make clear which outcome you anticipate. If two scientists expect exactly the same things to happen, they are in agreement, no matter how they call some particle. If you cannot make testable predictions, words begin to matter. This is where most philosophy starts, and this is what makes it hard to treat philosophy in a rigid scientific way. Sometimes philosophy makes claims that lie somewhere in between testable and purely philosophical [1]. “Sensations are brain-processes” is one of these and this paper examines it from a philosophical point of view.

This thesis, brought forward by the British philosopher and psychologist Ullin Place in 1956 and defended by the Australian philosopher John Smart in 1959, holds that, in contrast to behaviourism, mental states should not be identified with behavior but with neural states. When we are talking about what we feel, we are talking about a process in our brain — if we are talking about a process at all. According to Smart, a sensation is a brain-process in the same sense that lightning is an electric discharge. They are not only correlated, since correlation would imply that sensations are more than physical events. They are strictly identical: Two words describing the same thing.

Apply some common sense and you might think that sensations and brain-processes are not identical at all. I can talk perfectly well about my feelings even though I know next to nothing about neurobiology.

Yet it does not follow that I am talking about a process that is different from the processes neurobiology describes. A single phenomenon often has different levels of description. I can talk about clouds all day long and nobody will doubt that I am speaking about the same things chemists and physicists talk about when they discuss the behaviour of water molecules in the atmosphere.

It does, however, follow that — in the case of mental events — the psychic properties I am talking about are very different from the properties of the brain-processes natural science describes.

Furthermore, the “fact” that when we have a certain kind of sensation there is a certain kind of process in our brain is not a fact. It is a theory and as such, even if it is well-founded, subject to change. Therefore if we are talking about our sensations, what we mean cannot be our brain, since we do not know whether it is the brain that is responsible for our mental life.

Reality, however, is not shaped by what we mean. Our language contains a lot of words that have different meanings but nonetheless name the same things, and we are not always aware of the fact that two words refer to the same object. Thus even if brain-processes and sensations are identical, their properties which can be described from an introspective perspective are logically distinct from those described from a neurophysiological perspective.

These two arguments, the argument from our personal ability to talk about what we feel despite our lack of knowledge about neurobiology and the argument from the fact that we don’t mean our brain-processes when we talk about our feelings, make clear that there are two ways to describe our mental life and that it seems impossible to reduce one to the other. Besides Putnam’s argument from multiple realizability [2], this is one of the best objections to Smart’s identity theory of mind. It will turn out that this objection is not easy to reject.

What we have established so far is this: John Smart claims that brain processes are identical to sensations. We stumble upon the objection that the properties of brain processes appear to be very different from the properties of the events on a neurophysiological level. We note that, even if they appear to be very different, they may in fact be the same thing — think of the different ways to describe lightening, either with reference to electricity or with reference to the visual image of a flash in the sky. In any case, there are different levels of description and these descriptions seem to be by and large independent of each other.

Smart replies that the fact that sensations feel different from everything else is not reason enough to say that they are not identical with brain processes; if we were not scientifically advanced enough to know about the relation between electricity and the way lightening looks, we might be tempted to say that both follow a different logic. To show how the details of the relation between brain-processes and sensations look like is a task for neuroscience [3]. What remains to be determined here is whether the combination of brain-processes and the external world is sufficient to produce all the different kinds of sensations that we have, both those directly attributable to the real world and those that seem to live only in our mind.

In order to get a grip on the different types of phenomenal qualities that make up our sensations, Smart introduces the concept of primary and secondary qualities. Primary qualities are those qualities that are easily and distinctly detectable, e.g. the attribute of being a square. Secondary qualities are those qualities of objects, e.g. two objects being of the same shade of color, that have a special relation to our sensory powers: They can be detected only through a specific phenomenal experience. Since to what extent secondary qualities can be detected differs from person to person, they are not seen as representing simple distinctive attributes of the external, physical world. If properties like which colors you are able to differentiate from other colors are not attributes of the physical world, they have to be attributes of our mental world. If they are attributes of our mental world and if we want to show that our mental processes are identical to our brain processes, we have to be able to identify the sensations of these secondary qualities with brain processes.

In fact, both primary and secondary qualities are identifiable with brain processes. We can talk about both of them using topic-neutral words, that is, words that don’t tell us anything about whether physicalism or dualism is more likely to be correct. If I see a large blue area above myself, the description I give can be broken down to something like “There is something going on which is like what is going on when I look at the sky.” — It doesn’t matter whether I am actually looking at the sky or whether I am experiencing some kind of optical illusion. In both cases, the report given is a description of the processes that go on within my brain, not of the physical world. This is exactly what Smart’s version of the identity thesis claims: When I am talking about what I feel, I am talking about nothing else but a process in my brain.

Of course, none of the objections Smart responds to really matter compared to the much more fundamental problems that exist with the identity theory of mind. Identity applied to brain states and sensations means two things: On the one hand, each brain state is identical to exactly one experience [4]. Likewise, each phenomenal experience is identical to exactly one brain state. We cannot be sure that there are no two brain states that result in the same experience. In the process of looking at the most basic parts of identity theory, we might just as well ask: What constitutes a brain state? The configuration on the lowest physical level that is quite different every few seconds although my mental state feels quite the same to me? If yes, where does my brain end? Which molecules belong to my brain, which don’t? If brain state does not refer to the lowest physical level but instead to a pattern, then on which level?

The identity theory of mind does not have answers to most of these questions, neither does any other current theory of mind. Compared to other theories, Smart’s reductive approach doesn’t feel quite complete, especially concerning the way it mostly ignores the existence of qualia or leaves any explanation on this part to neuroscience. It will be exciting to see whether any of the theories of mind we know about today and which all seem to have their own specific problems will be declared the right one within the time of our lives.