PREFACE. THERE are three distinct modes of human knowledge, even when the subject of such knowledge remains the same, each of which is both valuable and necessary, but no one of which is perfect, either for use or for enjoyment, without the others. These three modes, or parts, if the word is preferred, are as follows: First, the knowledge of things and events, in their particular or individual characters and appearances. This is the foundation of the others; and, therefore, it must precede them, in the case of an individual, of a nation, or of the whole human race, as supposed to be in a state of progressive improvement. It is, as one would say, the foundation of all the rest; but, like other foundations, though there can be no structure without it, it is still the lowest part of the structure. The parts of it may be ever so many, or in themselves ever so valuable, but there is not connexion among them, and for this reason they are useless. The rolling pebbles on a shingly beach are, in all probability, composed of exactly the same materials as the solid rocks of the caverned shore, which breast or are bathed in the deep water. But while these sturdy rocks are fertile in sea-weeds, and peopled by innumerable animals, there b is not even a sprout of alga, or the smallest rudimental shell, upon the rolling pebbles of the shingly beach. In like manner, the earth of the fertile valley of Egypt, in as far as it is mere material earth, which has come directly from the rock, without passing through the action of life either vegetable or animal, is in all probability the same as that which forms the great desert of Sahara, and all the deserts in the north of Africa. But Egypt has from the earliest ages been famed for its fertility; and these deserts, wherever the name can be fully applied to them, are utterly barren of vegetation. Why, where the substantive materials of these places are exactly the same in substance, should there be this exuberance of life and growth in the one, and this perfect and unalterable sterility in the other? The question answers itself;-there is no bond of union-no means of connexion between pebble and pebble on the shingly beach, or between particle and particle of the sand on the desert; and therefore it is that the shingly beach and the sandy desert are barren of vegetation, and destitute of life. But, once let there come a connecting matter, a vinculum, or tie, of any kind, which shall unite the loose pebbles or the loose particles, and enable the element to which they are exposed to act upon them as upon one whole, and then the characteristic vegetation and life, which accords with the element, the climate, and the locality, will speedily make their appearance, and attain their perfection. In the case of that knowledge which applies only to individuals, it is exactly the same. It is barren of use fulness, because there is no vinculum-no connexion between the parts; and, therefore, it requires something in addition. This brings us to the other two modes: Secondly, the relations of the individual subjects of knowledge in their coexistence, or as they are found at any one time, without looking backward for those experiences which, used with judgment, enable us to look forward. This is, in its very nature, indefinite and inexhaustible: indefinite, because the extent of it depends on the number and nature of the subjects of individual knowledge possessed by the party to whom it is intended to be useful; and inexhaustible, because there is really no end to these subjects of knowledge. All, therefore, that can be done, so as to be adapted to the public generally, is really little more than showing the way by means of a few of the more general examples. This I have slightly attempted in the four small volumes on the "Heavens, Earth, Air, and Sea," the sale of which leads me to infer that they have been found not merely readable but useful; and it also encourages me to treat the remaining mode of knowledge in the same general and simple way; which attempt is embodied in the present volume, and in the other three which I intend shall follow it. This mode of knowledge is,— Thirdly, the relations of the subjects of knowledge in their succession in time, and chiefly as they follow each other as effects from causes. Some metaphysical subtilties have been raised about the relation of Cause and Effect; but they can hardly be regarded in any other light than that of ingenious mental trifling, the time spent on which would have been much more profitably devoted to something else;-for the parties entertaining them, always acted upon the very same principle of cause and effect as other men, which showed that their doubts and disputations on the subject were neither useful nor real. In this mode of knowledge, as in that of the relations of coexistence, little more can be done than showing the way. In this respect it is, indeed, more indefinite than the other. We can have testimony of much of the absent, and living testimony of not a little of it; but of the past in succession our living testimony is limited, and of the future we can have none whatever. But all our enjoyment is in the future, the past being irrevocably gone, and the present sliding away from us before we have time to question it. The future is, therefore, the portion of succession, in which all of us have the deepest interest; and to that,— in matters of human knowledge, upon which we have no direct revelation given,—we have really nothing to guide us but these very relations of succession, which we can obtain from the experience of the past, and from that only. This third mode of knowledge, therefore, is really the important one,-the one especially without which the others can be of no use. It cannot exist without some individual knowledge according to the first mode, and some knowledge of the relations of coexistence according to the second; but still, the ready application of a limited portion of knowledge in the way of cause and effect, tends to make |