Thus Newton, in spite of all his genius, cannot tell us what was the cause of the movement of the stars; he was not even able to determine the immediate mode of action of this cause; and yet there is not a scientific term more universally received than that of _Gravitation_, there is not a case in which the expression _Force_ is more generally accepted. The reason of this is, that in the presence of general facts and groups of phenomena, it is necessary to make use of terms as simple as possible. We must, however, avoid the delusion of thinking that _naming_ is equivalent to _explaining_. In cases analogous to that of which we have been treating, the word Force merely indicates the presence of an _Unknown Cause_, which gives rise to a _group of fixed phenomena_. In assigning names to each of the Forces or Unknown Causes to which we consider ourselves able to refer certain groups of phenomena, we facilitate the demonstration and discussion of the facts. The scientific man knows very well that he cannot go beyond this. It is in this sense, and in this sense alone, that I have used above the expressions _Force_ and _Life_. Astronomers consider gravitation the unknown cause of the movement of the stars; I consider Life as the unknown cause of the phenomena which are characteristic of organised beings. It may be that both gravitation and Life, as well as the other general forces are merely as _x_, of which the equation has not yet been discovered. I shall presently return to these considerations. Be this as it may, whatever our real ignorance, whatever the Cause of which we are here treating, and though _Impulsion_ should one day replace _Attraction_ in our Theories, the facts would still remain the same. The stars would still be distributed through space, and subject to the laws of Newton and Kepler; they would still constitute a perfectly distinct whole, in the part assigned to the bodies which compose it, and in the nature of the relations which unite them. They would still form the _Sidereal Kingdom_. This kingdom is then characterised by a _general phenomenon_, the _Keplerian Movement_, which may be attributed to a single force, namely that of _gravitation_. IV. Let us now return to the Earth, the only celestial body which we can study in detail. Modern discoveries, however, judging from the relation of the elements and their mutual action, make it almost certain that the greatest similarity exists between the stars distributed in space, between all those at least which form part of our heavens. Let us first establish the fact that upon our globe we again meet with the Keplerian Movement in falling bodies. Attraction is here represented by _Weight_. Gravitation reappears with all its laws, acting upon grains of dust as it acts upon worlds. The parts of the whole, of cosmos, as Humboldt would have said, cannot escape from the force which governs the whole.