Mind, Brain, and Time

Brain, Mind, and Time

Thoughts (and other mental events) occur in time and take time. Transitions between thoughts, as in logical reasoning, are temporally extended processes. Some people think more quickly than others. There is such a thing as the speed of thought, in principle measurable. Mental velocity is real. The brain also operates in time and has a speed. The speed of a nerve impulse (“conduction velocity”) is estimated as 275 mph (120 meters per second) at the high end and a few miles an hour at the low end (it depends on axon diameter and myelination). Given the dependence of the mind on the brain, then, thought speed cannot exceed brain speed, even though it may feel instantaneous. If the mind were not thus dependent, this would not necessarily be so—the mind might be able to go faster than the brain. As it is, however, the speed of the brain constrains and controls the speed of the mind. The machinery of thinking is housed in the brain and it operates according to strict rules. The nerve impulse is much slower than electricity, though faster than the flow of blood, which allows thought to proceed at a fair lick; but it cannot break free of the physical dynamics of the brain. The brain sets an upper limit on mental velocity.

We can infer from this that mental time is the same as physical time: the velocity of thought formation is identical the velocity of neural transmission, roughly speaking. I don’t mean it can’t be slower; I mean that there is no separate time in which mental events occur. In fact, the time between two successive thoughts is the same as the time between their neural correlates. Mental velocity is derivative from physical velocity, a special case of it. Thus, a modest physicalism is true of the speed of thought; it is the speed of the corresponding neural transition, neither more nor less. It is not something separate from, and over and above, brain speed. As brain scientists say, they have the same latencies. Mental velocity is supervenient on physical velocity precisely because it is physical velocity. If the mental machinery were made of something else, the speed of thought could be different, either slower or faster (AI could be much faster). If you ask what is the speed of thought as such, you get no answer; it has no intrinsic natural speed—it all depends on the nature of the relevant brain machinery. If the brain were made of light, its thoughts might reach the speed of light; if it were made of a liquid like blood or molasses, it might be as slow as walking speed. The speed of thought is essentially a physical thing.

Normally, if two entities are separate and distinct, the speed of one does not depend on the speed of the other—for example, two cars moving down the highway. If the things are identical, however, then their speed must be the same (by Leibniz’s law). Clark Kent can fly as fast as Superman; water boils at the same rate as H2O. If the things are joined in some way, glued together, they will move at the same speed, but if disconnected they will be able to move at different speeds. The mind cannot move at a different speed from its associated brain, so there is really only one explanation of its tracking of brain dynamics—it must be the correlated brain state. Why do the two march exactly in step? Because one is the other; the mind isn’t a separable entity capable of moving at a different velocity from its physical basis. In other words, your thoughts are in your brain—that’s why the two proceed through time at the same rate. They may have properties over and above regular brain properties, but they are literally parts or constituents of your brain, as neurons are. We might have concluded this on independent grounds, but the consideration of mental velocity has provided us with another reason for taking this view. The best explanation of temporal coincidence is spatial coincidence—as with the Superman and water cases. This tells us nothing about subjective properties of mental states, but it does tell us that those states are identical to states of the brain described in neural terms. The thought in time is none other than its neural correlate also in time. Mental events are physical events because their succession in time is identical to the succession in time of their brain correlates. They can’t proceed in time independently of the brain, and this must be because they are parts of the brain. A dualist view would imply that the two temporal sequences could come apart, but they can’t and don’t. The mind is tied tightly to the brain in its dynamic aspect, even if it might have other aspects that are not so tied.[1]

[1] It might be said that we have a proof of token identity but not type identity, and this wouldn’t be wide of the mark. The proof proceeds from premises concerning the speed of thought (not, say, from premises about causation and laws, a la Davidson). Mental events must be physical events because of their temporal indiscernibility.