But various other influences, and among them those usually designated as chances, exert a powerful influence in changing the order of nature. It is necessary that its forces should not only be combined, but combined at the right time and place, or the effect which is due to their combination will not take place; _i.e._ a different order of natural events would have happened. An illustration will make this clear. Let us take the case of a disintegrating rock; according as the different forces, which act on it, meet at the suitable time and place, the progress of disintegration is greatly lengthened or shortened. Such concurrences of events are what we view as pure contingencies. _E.g._ water penetrates into one of its fissures; this takes place in summer, and no appreciable result follows. But if in winter a frost happens immediately afterwards, it will produce an order of events widely different from that which would have happened if either no rain had fallen or frost occurred. By their joint agency the fissure is widened, or the rock split asunder. It follows therefore that the concurrence of these two forces is necessary at a particular time and place to produce the particular result. Such concurrences, though due to natural causes, are what we call fortuitous. Yet their occurrence or non‐ occurrence occasions a different order of natural events. Further, let us suppose that a bird with a seed in its mouth, in the course of its flight casually drops it into a fissure in the rock, which has been opened by the frost; and also that another concurrence of forces has supplied the conditions suitable for its taking root and growth. This produces a new series of events, which occasions a more rapid disintegration, and modifies the whole of the results which follow. If the casual act of the bird had taken place at any other time or place, the whole series would have been different, varying with the causes which produced the seed, and the contingencies which brought the bird to the spot, and induced it to drop it. Let us now suppose that man with his rational agency intervenes. He deliberately watches for the prospect of a frosty night, pours water into the fissures, and plants seeds in fissures where he knows that suitable material has been prepared for their growth. Here a new order of events has been introduced, which, originating in human agency, entirely modify the order of the results.