We say commonly that we think with our brains. That is true; but it is by no means the whole story. The brain has two halves, just alike, exactly as the body has. In fact, the two sides of the brain are even more precisely alike than the two hands.

Nevertheless, we do all our thinking with one side only. If we are naturally right-handed persons, we do our thinking on the left side of the head. If we are naturally left-handed, we do our thinking on the right side. But we do not use both sides. Each half of the brain governs the muscles of one side of the body; but the thinking is done with one side only.

This very peculiar state of affairs is in some way or other connected with our power of speech. The animals, who cannot use words, do their thinking, so far as we know, equally on both sides of their heads; and we have every reason to believe that if we did not talk, we too should do our thinking with both sides.

Let us see if we cannot in part make out the reason for this arrangement. Let us suppose you are using your mouth and tongue, not to talk with, but to eat your breakfast. Each half of the brain, as I have said, controls the muscles on one side of the body; and as I dare say you have long ago been taught in school, a set of long nerve fibers, like so many telephone lines, connects each muscle with the proper region of the outer surface of the brain. You are, we are supposing, at breakfast, and you take a bite of bread and butter. At once, the two halves of the tongue telephone by way of the nerves, each to its own side of the brain, “Something good to eat, shall we chew and swallow?” Then each half-brain telephones back, “Message received, stand by to chew, on signal; will call up other side.” Then each side of the head rings up the other side and says, “Bread and butter reported in my half the mouth; shall chew at once. Are you ready to start? Go!” And away go the two sides of the face working together.

Now of course I do not mean to say that this is literally what occurs every time you attempt to do anything that requires both sides of the body to act together. Nevertheless, something like this really does happen. You know how a tiny baby cannot even look at the same point with both eyes at once, but sends them straddling off, one looking one way, the other another. It takes some days for the two halves of the baby’s brain to learn to consult one another, and to handle their two eyes in co-operation. So with any act that uses two hands, or two feet, or a hand on one side of the body and a foot on the other. The two halves of the brain have to call across to one another, along certain nerve fibers which run back and forth between the two, in order that the actions of the two sides of the body may keep together. You can easily see that if one side of the brain tried to open its side of the mouth, while the other side was trying to shut its, you would probably have to start for school without your breakfast.

Naturally, after the two brain halves have been living together for a few years, growing up together in the same house, they learn to work almost like one. Still this signaling back and forth does take time. So long as we are merely eating, or walking, or shoveling snow, the process goes on fast enough. But talking is a different matter. When you are talking thirteen to the dozen, just as fast as you can chatter, every several letter of every single word you utter demands at least one change of position of tongue or lips or throat, and generally of them all.

What would happen if the two halves of the brain had to stop to call one another up and say, “Now I am on the point of starting to say ‘t’ with my half of the mouth. Are you ready with yours?” “Now I am going to tuck on an ‘h.’ Are you ready with your side?” “Now go ahead with the ‘e.’ Start.” It wouldn’t do at all. It’s altogether too slow a way to get talking done. So by way of saving time, one side of the brain has taken entire charge of the talking; for this, one side only of the brain runs both sides of the mouth.

When we eat, then, both sides of the brain are in action. But when we use the very same muscles for speaking, then we use one side of the brain only. When we lift a weight with both hands, we signal to the muscles from both halves of the brain. But when we play the piano with both hands, the same side of the brain takes charge of both. I am, for example, using a type-writer, and writing with both hands. Only one half of my brain, however, has charge of the writing. The other half simply side-tracks itself, stands aside, and doesn’t meddle. But the minute I stop writing and start to put my machine in its case, then the other half-brain switches on again, and takes care of its side. If I should hurt my right hand, so that I had to do all my writing with the left, the writing side of my brain would still do all the writing, while the other side that commonly manages my left hand would stand and look on. All these very special, complex, rapid and difficult tasks, like talking, writing, playing the piano, or running the type-writer, are done by one side of the brain. The slow and easy things are done by both.

But the animals, who do not either talk or write or play musical instruments, they use the two sides of their brains alike.