(95.) The forces which are thus in constant operation, proceed from secret agencies which the human mind has never been able to detect. All the analogies of nature prove that they are not the immediate results of the divine will, but are secondary causes, that is, effects of some more remote principles. To ascend to these secondary causes, and thus as it were approach one step nearer to the Creator, is the great business of philosophy; and the most certain means for accomplishing this, is diligently to observe, to compare, and to classify the phenomena, and to avoid assuming the existence of any thing which has not either been directly observed, or which cannot be inferred demonstratively from natural phenomena. Philosophy should follow nature, and not lead her. While the law of inertia, established by observation and reason, declares the inability of matter, from any principle resident in it, to change its state, all the phenomena of the universe prove that state to be in constant but regular fluctuation. There is not in existence a single instance of the phenomenon of absolute rest, or of motion which is absolutely uniform and rectilinear. In bodies, or the parts of bodies, there is no known instance of simple passive juxtaposition unaccompanied by pressure or tension, or some other “tendency to motion.” Innumerable secret powers are ever at work, compensating, as it were, for inertia, and supplying the material world with a substitute for the principles of action and will, which give such immeasurable superiority to the character of life. (96.) The forces which are thus in continual operation, whose existence is demonstrated by their observed effects, but whose nature, seat, and mode of operation are unknown to us, are called by the general name _attractions_. These forces are classified according to the analogies which prevail among their effects, in the same manner, and according to the same principles, as organised beings are grouped in natural history. In that department of natural science, when individuals are distributed in classes, the object is merely to generalise, and thereby promote the enlargement of knowledge; but nothing is or ought to be thus assumed respecting the essence, or real internal constitution of the individuals. According to their external and observable characters and qualities they are classed; and this classification should never be adduced as an evidence of any thing except that similitude of qualities to which it owed its origin.