When a stone falls into the water and causes ripples on the surface, the stone is no more the cause of the ripples than the liquid condition of the water. If the stone falls on solid substances, it causes no ripples. It is the contact of the falling stone with liquid substances which causes the ripples. The cause is itself an effect, and the effect, the ripples, become a cause when they carry a piece of cork ashore. But in either case the cause is based on a mutual effect, on the interaction of the waves with the light condition of the cork. A stone falling into the water is not a cause "in itself," not a cause in general. We arrive at such a cause only, when the faculty of thought uses concrete causes for its raw material and constructs out of them the "pure" concept of the cause in general. A stone falling into the water is only the cause of the subsequent ripples, and it becomes a general cause only through the experience that ripples always follow the falling of a stone into water. We call cause that which generally precedes a certain manifestation, and effect that which generally follows it. We refer to the stone as the cause of ripples merely because we know that it always causes them when falling into water. But since ripples sometimes appear without being preceded by the fall of a stone, ripples have another general cause. So far as there is anything general in ripples which precedes them, it is the elasticity of the water itself which is the general cause of ripples. Circular ripples, which are a special form of ripples, are generally preceded by the falling of some body into the water, and this body is then considered as their cause. The cause is always different in proportion and to the extent of the phenomena under consideration. We cannot ascertain causes by mere introspective reasoning, we cannot derive them out of our head. Matter, materials, sense perceptions are required for this purpose. A definite cause requires a definite material, a definite amount of sense perceptions. In the abstract unity of nature, the variations of matter are represented by the variations of concrete quantities. Every quantity is given in time before and after a certain other quantity, as antecedent and subsequent. The general element of the antecedent is called cause, the general element of the subsequent, effect.