History helps us to understand the transition which led from one to the other. Ancient philosophy made the earth the centre of the universe. Notwithstanding the diversity of their particular characteristics and of their motions, it was natural then for all the celestial bodies to be conceived as the parts of a single system. A more or less clearly expressed postulate supported this astronomical conception: the purpose of the universe was the existence of man. There was no occasion to distinguish our world from the whole world. But could this conception stand when the earth was reduced to the condition of a planet revolving round a sun so like a multitude of other suns. Suddenly the stars were carried to distances infinitely more considerable than the greatest planetary intervals. Undoubtedly the human mind could continue to regard the very small groups of which the earth forms a part as a system. But the system (if it exists) which embraces the whole of the celestial bodies ceased henceforth to be within our reach. Since then “the notion of the world has become clear and habitual, and that of the universe has become uncertain and almost unintelligible.”[125] It matters little, moreover, for, according to one of Comte’s favourite maxims, what we have no means of knowing, neither have we any need to know; and every thing which it is our interest to learn we can also attain. Nor should we see in this any providential harmony. That which it is our interest to know must always in some way influence the conditions of our existence. By the mere fact that this action makes itself felt, it is inevitable that sooner or later, directly or indirectly, we should come to know of it. This reflection can be well applied to astronomy. The study of the laws of the solar system, of which we form a part, is of supreme interest for us: and we have reached very great precision on this point. On the contrary, the exact notion of the universe is inaccessible to us; but it is unimportant to us leaving out of the question our “insatiable curiosity.” The independence of our world is certain. The phenomena which take place within the solar system do not appear to be affected by the more general phenomena which relate to the mutual action of suns. Our tables of celestial events, drawn up long beforehand and taking into consideration no other world than our own, so far accord strictly with direct observations. Supposing the law of gravitation to extend to the entire universe, the perturbation in our world caused by a mass equal to a million times its own, and which would be situated at the distance of the nearest sun to our own, would be several thousand million times less than that which brings about our tides, that is to say practically nil.