The existence, then, of matter is a fact: the existence of motion is another fact. Our visual organs point out to us matter with different essences, forming a variety of combinations, endowed with various properties that discriminate them. Indeed, it is a palpable error to believe that matter is a homogeneous body, of which the parts differ from each other only by their various modifications. Among the individuals of the same species that come under our notice, no two resemble exactly; and it is therefore evident that the difference of situation alone will, necessarily, carry a diversity more or less sensible, not only in the modifications, but also in the essence, in the properties, in the entire system of beings. This truth was well understood by the profound and subtle LEIBNITZ. If this principle be properly digested, and experience seems always to produce evidence of its truth, we must be convinced that the matter or primitive elements which enter into the composition of bodies, are not of the same nature, and consequently, can neither have the same properties, nor the same modifications; and if so, they cannot have the same mode of moving and acting. Their activity or motion, already different, can be diversified to infinity, augmented or diminished, accelerated or retarded, according to the combinations, the proportions, the pressure, the density, the volume of the matter, that enters their composition. The endless variety to be produced, will need no further illustration than the commonest book of arithmetic furnishes us, where it will be found, that to ring all the changes that can be produced on twelve bells only, would occupy a space of more than ninety-one years. The element of fire is visibly more active and more inconstant than that of earth. This is more solid and ponderous than fire, air, or water. According to the quantity of these elements, which enter the composition of bodies, these must act diversely, and their motion must in some measure partake the motion peculiar to each of their constituent parts. Elementary fire appears to be in Nature the principle of activity; it may be compared to a fruitful leaven, that puts the mass into fermentation and gives it life. Earth appears to be the principle of solidity in bodies, from its impenetrability, and by the firm coherence of its parts. Water is a medium, to facilitate the combination of bodies, into which it enters itself, as a constituent part. Air is a fluid whose business it seems to be, to furnish the other elements with the space requisite to expand, to exercise their motion, and which is, moreover, found proper to combine with them. These elements, which our senses never discover in a pure state--which are continually and reciprocally set in motion by each other--which are always acting and re-acting, combining and separating, attracting and repelling--are sufficient to explain to us the formation of all the beings we behold.