"Suppose there existed a Tantalus who was condemned for evermore to strike with a hammer upon an anvil. Suppose that Tantalus, his hammer, and his anvil were concealed from the observer’s view by a screen or otherwise, and that a light, carefully arranged, threw the shadow of the hammer and anvil upon a wall where it could easily be seen.

Suppose an observer, whose mind was tabula rasa [a blank tablet] were set to watch the shadow. Every time the shadow of the hammer descended upon the shadow of the anvil, the sound of the percussion is heard. The sound is only heard when the two shadows meet. The hammer’s shadow occasionally beats fast, occasionally slow: the succession of sounds exactly corresponds. Perhaps the hammer raps out a tune on the anvil; every note heard follows upon a blow visible in the shadows. The two series correspond invariably and absolutely; what is the inevitable effect upon the observer’s mind? He knows nothing of the true cause of sound behind the screen: his whole experience is an experience of shadows and sounds. He cannot escape the conclusion that the cause of each sound is the blow which the shadow of the hammer strikes upon the shadow of the anvil.

The observer is in the position of an introspective philosopher. Introspection teaches us nothing about nerve currents or cerebral activity: it speaks in terms of mind and sensation alone. To the introspective philosopher, it is plain that some mental or psychical process is the condition of action. He thinks, he feels, he wills, and then he acts. Therefore the thinking and feeling and willing are the cause of the acting. Introspection can get no farther. But now the physiologist intervenes. He skillfully dissects away the screen and behold! there is a real hammer and a real anvil, of which nothing but the shadow was formerly believed to exist. He proves that states of consciousness are shadows accompanying cerebral functioning; he shows that the cause of action lies in the cerebral functioning and not in the shadows which accompany it."