Abstract

Once upon a time, there were two large black boxes, A and B, connected by a long insulated copper wire. On box A there were two buttons, marked *a* and *b*, and on box B there were three lights, red, green, and amber. Scientists studying the behavior of the boxes had observed that whenever you pushed the *a* button on box A, the red light flashed briefly on box B, and whenever you pushed the *b* button on box A, the green light flashed briefly. The amber light never seemed to flash. They performed a few billion trials, under a very wide variety of conditions, and found no exceptions. There seemed to them to be a causal regularity, which they conveniently summarized thus