The idea of explaining every phenomenon in nature by mechanical principles was certainly a great and beautiful exertion, and which Descartes first attempted. But this idea is only a project, and if properly founded, have we the means of performing it? These mechanical principles are the extent of matter, its impenetrability, its motion, its external figure, its divisibility, and the communication of movement by impulsion, by elasticity, &c. The particular ideas of each of these qualities we have acquired by our senses, and regard them as principles, because they are general and belong to all matter. But are we certain these qualities are the only ones which matter possesses, or rather, must we not think these qualities, which we take for principles, are only modes of perception; and that if our senses were differently formed, we should discover in matter, qualities different from those which we have enumerated? To admit only those qualities to matter which are known to us, seems to be a vain and unfounded pretension. Matter may have many general qualities which we shall ever be ignorant of; she may also have others that human assiduity may discover, in the same manner as has recently been done with respect to gravity, which alike exists in all matter. The cause of impulsion, and such other mechanical principles, will always be as impossible to find out as that of attraction, or such other general quality. From hence is it not very reasonable to say, that mechanical principles are nothing but general effects, which experience has pointed out to us in matter, and that every time a new general effect is discovered, either by reflection, comparison, measure, or experience, a new mechanical principle will be gained, which may be used with as much certainty and advantage as any we are now acquainted with? The defect of Aristotle's philosophy was making use of particular effects as common causes; and that of Descartes in making use of only a few general effects as causes, and excluding all the rest. The philosophy which appears to me would be the least deficient, is that where general effects are only made use of for causes, and seeking to augment the number of them, by endeavouring to generalize particular effects. In my explanation of expansion and reproduction, I admit the received mechanical principles, the penetrating force of weight, and, by analogy, I have strove to point out that there are other penetrating powers existing in organized bodies, which experience has confirmed. I have proved by facts, that matter inclines to organization, and that there exists an infinite number of organic particles. I have therefore only generalized some observations, without having advanced any thing contrary to mechanical principles, when that term is used as it ought to be understood, as denoting the general effects of Nature. CHAPTER IV. OF THE GENERATION OF ANIMALS.