The unity of the soul is an ancient doctrine from which we have inherited a strong belief in the unity of consciousness. In certain lights this assumption of unity seems unquestionable, but it has actually been a continual problem; it could almost be argued that the history of understanding the mind has been a history of giving up on unity.
Like other persuasive doctrines that have turned out to be problematic in the long run, we can trace this one back to Aristotle, but it is tied in to a widely-held set of scholastic/ancient ideas about metaphysics. I believe the argument runs more or less like this: the soul is not physical, therefore it lacks extension (which is a physical property); if it lacks extension it necessarily lacks parts, and if it lacks parts it must be single and unified. The soul is a substance, in the old philosophical sense of something incapable of being analysed or broken down. Substances in this sense used to be considered necessary building blocks of reality, required in order to have a secure ontological foothold. Otherwise the process of analysis would be bottomless and unending, and nothing would ever be completely clarified, which would be intolerable (although I notice contemporary physics seems to tolerate a position not altogether unlike this). Readers may well by now feel parts of their own souls waving urgent hands to attract attention to a host of salient objections, but let’s avoid getting bogged down in this treacherous territory and move on a bit.
Descartes, say what you will about his dualism, effected a radical change for the better when he restricted the interventions of the soul to the pineal gland: on his view it did its stuff there and the rest of the body worked like a machine, according to the same physical laws as any inanimate stuff. Until then it had been largely assumed that the soul directly activated the body without needing any kind of transmission mechanism. Now I say ‘until then’, but the remarkable fact is that people went on thinking that way for a long time afterwards. As late as 1850, Helmoltz’s measurement of the speed of nerve impulses was resisted by some on the grounds that the vital impulse must act throughout the body simultaneously. When your arm moved, it was because you wanted it to, and it, as part of you, wanted to too. I believe there was a school of thought that held out for a middling point of view, accepting that in principle the brain controlled the body by nerve impulses, but confidently expecting that they would be too blindingly fast to ever be measured. This, of course, proved to be quite wrong, but the nineteenth-century debate is in some ways quite reminiscent of the more recent discussion of Libet. Muller and others thought Helmholtz must be wrong because he was introducing a delay between will and act; people suppose Libet must be wrong because he introduces a delay between deciding to act and awareness. All such delays are intolerable if we insist on the absolute unity of the conscious mind because you can’t have a delay between a thing and itself.
Another prominent example of the problems flowing from unity is the vexed issue of the binding problem. Given that sensory inputs come in by different pathways at different speeds and get processed in different ways in different parts of the brain, how is it we end up with a smoothly integrated picture of reality which assigns the right qualities to the right objects and unrolls steadily in real time without jumps, pauses, or lipsynch errors? There are various ways, more or less satisfactory or problematic, in which the brain might ensure everything is properly put together when it arrives in consciousness, but if we’re not assuming consciousness is a single united destination, the problem wouldn’t arise in the first place.
Perhaps, though, the binding problem gives us a clue about why unity seems so undeniable – because the contents of consciousness look united. Isn’t that it?
Well, sort of, but when I sit down and conscientiously introspect, I don’t really detect a lot of unity. At the moment I have strings of explicit words running through my mind a moment before I type them: I moment ago as I sat in uffish thought, I had thoughts about the same subject which were wordless. Half an hour ago I wasn’t thinking about anything at all, though I was certainly conscious, and a bit before that I was largely absorbed by experiencing the taste of scrambled egg. An hour before that I was dreaming and some time before that in a blank state of which I can’t say for sure whether I remember it or not.
It’s worse than that, because at all these times there were also things in the penumbra of my mind which I was aware, or perhaps only pre-aware or potentially aware of. Hume famously said that when he looked into his mind he found only a bundle of sensations; but how simple it would be if the sensations were really always bundled; if they were always of the same broad kind; and if they were all merely sensations, instead of including bits of broken intentionality, fragments of half-or potentially meaningful intimations, things that might be the phenomenally detectable end of affordances, incipient recognitions and implicatures and an exquisitely ineffable and shadily located intimation that there may soon be the emergence of what we can call a gut feeling finely balanced on the cusp between the affective and the merely digestive. A bundle? Really a heap, or even a cloud, would be far more orderly and unified than my subjective experience.
What does bind things together is a kind of bird’s nest framework of memory linking now to then, and then to some other experience, and so on; but this is not all that useful. For one thing these linkages are loose – what they provide is the kind of tangled and ad hoc unity the cables behind my stereo have achieved unbidden – and they are fallible, not part of the essence of consciousness. I see nothing absurd about the idea of my having moments of consciousness which are neither remembered nor involve remembering.
Yet even so I find it intuitively impossible to abandon the idea of some unity somewhere, even if I can’t quite put my finger on it at the moment. Things would be so much easier if we didn’t exist, but there we are.