Often two events are thought of as causally linked. A set of conditions labeled A causes an effect B to occur. From a physical point of view, Newton's second law, F = ma, contains the idea that forces create accelerations. The change in the state of an object results as a consequence of those forces that act upon the object. Something that is not influenced by such forces remains unaffected, but once the object realizes the set of conditions A, it necessarily follows that the action B must occur. In other words, whenever A is satisfied, then the event B cannot fail to occur. An example illustrates the point. Consider a cue ball striking a stationary billiard ball. The cue ball imparts some of its momentum to the billiard ball, which was initially at rest, and the second ball starts to roll. The collision of the cue ball is...