(Discusses the interaction between physical and mental things, and the possibility of freedom in a world of fixed causes.) It is our plan, so far as possible, to discuss the problems of the mind in one section of this book, and the problems of the body in another; but just as we found that we could not separate our duties to ourself from our duties to our neighbors, so we find that the mind and the body are inextricably interwoven, and that whenever we probe deeply into one, we discover the other. The interaction of the mind and the body is a fascinating problem into which we must look for a moment, not because we expect to solve it, but because it illuminates the whole subject.
The human body is a machine. It takes in carbon and oxygen, and burns them, and gives out carbon dioxide and other waste products, and develops energy in proportion to the amount of carbon it consumes. This machine has its elaborate apparatus of action and reaction, its sensory organs where outside stimuli are received, its nerves like telegraph wires to carry these impressions, its brain cells to store them and to transform them into reactions. We know to some extent how these brain cells work. We know what portions of the brain are devoted to this or that activity. We know that if we stick a pin into a certain spot we shall paralyze the left forefinger. We know that by injecting a certain drug, or by breathing a certain gas, we can cause this or that sensation or reaction, such as laughing or weeping or mania. We know what poisons are generated in the system by anger, and what chemical changes take place in a muscle that is tired. All this is part of a vast new science which is called bio-chemistry, or the chemistry of life.
Our bodies, therefore, are part of the material universe, and subject to the laws or ways of being of this universe. The first of these laws that we know is the law of causation. Every change in the universe has its cause, and that in turn had another cause; this chain is never broken, no matter how far we go, and the same causes universally produce the same effects. If you see a ball move on a billiard table, you know that the ball did not move itself; you know that something struck the ball or tilted the table. You discover that the motion of the ball moves the air around it, and the waves of that motion are spread through the room. They strike the walls, and the motion is carried on through the walls, and if we had instruments sensitive enough, we could feel the motion of that billiard ball at the other side of the world, and a few million years from now at the most remote of the stars. This is what is called the law of the conservation of energy, and when we discover something like radium which seems to violate that law by giving out unlimited quantities of energy, we investigate and discover a new form of energy locked up in the atom. In the disintegration of the atom we have a source of power which, when we have learned to use it, will multiply perhaps millions of times the powers we are now able to use on this earth. But energy, no matter how many times it is transformed, and in what strange ways it reappears, always remains, and is never destroyed, and never created out of nothing.
My friend the great physiologist once took me into his laboratory and showed me a little aquarium in which some minute creatures were wiggling about—young sea-urchins, if I remember. The physiologist took a bottle containing some chemical, and dropped a single drop into the water, and instantly all these little black creatures, which had been darting aimlessly in every direction through the water, turned and swam all in one direction, toward the light. They swam until they touched the walls of the aquarium, and there they stuck, trying their best to swim farther. "And now," said my friend, "that is what we call a 'tropism,' and all life is a tropism. What you see in that aquarium means that some day we shall know just what combination of chemicals causes a human being to move this way or that, to do this thing or that. When bio-chemistry has progressed sufficiently, we shall be able to make human qualities, perhaps in the sperm, perhaps in the embryo, perhaps day by day by means of diet or injection."
Said I: "Some day, when bio-chemistry has progressed far enough, you will know what combination of chemicals causes a man to vote the Democratic or Republican ticket."
"Why not?" answered my friend. (He has a sense of humor about all things except this sacred bio-chemistry.)
Said I: "When you have got to that stage, keep the secret carefully, and we will fix up a scheme, and a few days before election we will release some gas in our big cities, and sweep the country for the Socialist ticket."
But jesting aside: if the human body is a material thing, existing in the material world and subject to causation, there must be material reasons for the actions of human bodies, just the same as for the moving of billiard balls. We hear the sound of a billiard ball striking the cushion, and we are prepared to accept the idea that the thing we call hearing in us is caused by the impinging of sound waves upon our eardrums. And if we investigate human beings in the mass, we find every reason to believe that they act according to laws, and that there are material causes for their acts. If you get up and shout fire in a theater, you know how the audience will behave. If you study statistics, you can say that in any large city a certain fixed number of human beings are going to commit suicide every month; you can even say that more are going to commit suicide in the month of June than in any other month. You can say that more people are going to die at two o'clock in the morning than at any other hour. You know that certain changes in the weather will cause all human beings to behave in the same way. You know that an increase of prices or an increase of unemployment will cause a certain additional number of men to commit crimes, and a certain additional number of women to become prostitutes. You know that if a man overeats, his thoughts will change their color; he will have what he calls "the blues." I might cite a thousand other illustrations to prove that human minds are subject to material laws, and therefore to investigation by the bio-chemists.
But now, stop a moment. Here you sit reading a book. Something in the book pleases you, and you say, "Good!" Perhaps you slap your knee or clench your fist. Now here is a motion of your hand, which stirs the air about you, and which, according to the laws of energy, will spread its effects to the other side of the world, and even to the farthest of the stars. Or perhaps the book makes you angry, and you throw it down in disgust; an entirely different motion, which will affect the other side of the world and the farthest of the stars in an entirely different way. The machine of the universe will be forever altered because of that slapping of your knee or that throwing down of your book.
And what was the cause of these things? So far as we can see, the material cause was exactly the same in each case—the reading of certain letters. Two human beings, sitting side by side and reading exactly the same letters, might be affected in exactly opposite ways. It seems hardly rational to maintain that the material difference of two pairs of eyes, moving over exactly the same set of letters, could have resulted in two such different motions of the hands. As a matter of fact, the very same letters may affect the same person in different ways. The composer, Edward MacDowell, once told me how on his birthday his pupils sent him a gift, with a card containing some lines from the opera "Rheingold," beginning, "O singe fort"—that is, "Oh, sing on." But the composer happened, when glancing at the card, to think French instead of German, and got the message, "Oh, powerful monkey!" This, of course, was disconcerting to a famous piano performer, and his pupils, if they had been watching his face, would have seen an unexpected reaction. It seems manifest, does it not, that the cause of this difference of reaction was not any difference of the letters, but purely a difference of thought? So it appears that thoughts may change the material universe; they may break the chain of causation, and interfere with material events.
Compare the two things, a state of consciousness and say, a steam shovel. They are entirely different, and so far as we can see, entirely incompatible and unrelated. Can anyone imagine how a thought can turn into a steam shovel, or a steam shovel into a thought? We can understand how a steam shovel lifts a mass of earth out of the ground, and we can understand how a human hand moves a lever which causes the shovel to act; but we are unable to conceive how a state of mind—whether it be a desire for pay, or an ideal of service, or a vision of the Panama Canal—can so affect a steam shovel as to cause it to move. We can sit and think motion at a billiard ball for a thousand years, and it does not move; but when we think motion at our hand, it moves instantly, and passes on the motion to the billiard ball or the steam shovel. When fire touches our hand it sends some kind of vibration to the brain, and in some inconceivable way that vibration is turned into a state of consciousness called pain, and that is turned, "as quick as thought," into another kind of motion, the jerking back of our hand.
So it seems certain that consciousness really does "butt in" on the chain of............