The proposition that causes and effects are recognized, not by the understanding but because of experience, will be readily granted if we think of such things as we may recollect we were once altogether unacquainted with. Suppose we give a man who has no knowledge of physics two smooth marble plates. He will never discover that when laid one upon the other they are hard to separate. Here it is easily observed that such properties can be discovered only through experience. Nobody, again, has the desire to deceive himself into believing that the force of burning powder or the attraction of a magnet could have been discovered a priori. But this truth does not seem to have the same validity with regard to such processes as we observed almost since breath began. With regard to them, it is supposed that the understanding, by its own activity, without the help of experience can discover causal connections. It is supposed that anybody who is suddenly sent into the world will be able at once to deduce that a billiard ball will pass its motion on to another by a push. But that this is impossible to derive a priori is shown through the fact that elasticity is not an externally recognizable quality, so that we may indeed say that perhaps no effect can be recognized unless it is experienced at least once. It can not be deduced a priori that contact with water makes one wet, or that an object responds to gravity when held in the hand, or that it is painful to keep a finger in the fire. These facts have first to be experienced either by ourselves or some other person. Every cause, Hume argues therefore, is different from its effect and hence can not be found in the latter, and every discovery or representation of it a priori must remain voluntary. All that the understanding can do is to simplify the fundamental causes of natural phenomena and to deduce the individual effects from a few general sources, and that, indeed, only with the aid of analogy, experience, and observation.