The use of hypothesis, as it is employed in astronomy, must be carried into the other sciences. This mode of procedure everywhere remains like to itself, although we do not always conceive it so clearly. “Its normal domain coincides with that of observation.” An hypothesis completes by anticipation what we know of facts and of their laws. Consequently, it is subject to be modified, corrected, or contradicted by a wider or deeper knowledge of facts. Hypotheses then are only valid during the time when they are advantageous, that is to say, as long as they serve to unite and co-ordinate our observations. As has been said, they labour to render themselves useless. But they are indispensable, and science, without them, could neither advance nor even begin. Far from giving too small a share to hypothesis, like Bacon, Comte would rather incur the reproof of having given it too large a one. He made too much use of it himself at the end of his life. But the theory which he gave of it in the Cours de philosophie positive and of which certain features appeal again in Claude Bernard’s Introduction a l’étude de la médecine experiméntale, was a careful study of its nature and function.
III.
Astronomy, or at least that part of astronomy which bears the name of celestial mechanics, of all the physical sciences is the one which has been carried to the highest degree of perfection. Nowhere else have the phenomena been better reduced to a supreme law which allows us to foresee them with sufficient precision. But this result could only have been obtained by substituting the notion of a solar world to that of a universe.[123] This world is the only one which we can comprehend as a system. If the object of astronomy were the general laws of the universe, this science would be extraordinarily imperfect, not to say impossible. For what do we know about cosmic laws?[124] We do not even know whether Newton’s law applies to any or all systems of stars.
We must then distinguish between astronomy as the science of our world and sidereal astronomy. The latter is not absolutely forbidden us, but we know very little on this subject, and we shall probably never know much more. Do the innumerable suns scattered in space form a general system, or do independent systems exist? Is space limitless? Is the number of celestial bodies an infinite one? philosophers ask. In truth the consideration of our world is positive. The consideration of the universe is not.
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.
Here, says Comte, is the only exception to the encyclopædic law according to which the more general phenomena control the more particular ones without being influenced by them.[126] From this he simply concludes that the phenomena of our system are the most general to which positive research can extend, and that the study of the universe must henceforth be excluded from natural philosophy. The encyclopædic law then remains true for the whole of positive philosophy.
The delimitation of the object of astronomy is one of the points where we can best follow the successive modifications of Comte’s thought. In the second volume of the Cours de philosophie positive he gave to astronomy the place which is generally conceded to it by scientific men. He even claims, as a condition for its utility, the most perfect disinterestedness of scientific research in the whole extent of its province. The example which he gives of it (the determination of longitude at sea), is borrowed from Condorcet. Undoubtedly, Comte already insists upon the distinction between the ideas of world and universe, the former only being positive. Nevertheless, he still admits that we should not give up all hope of obtaining some sidereal knowledge,[127] and that it would be very precious for us to know the relative motions of multiple stars, etc. But already in the sixth volume of the Cours he condemns entirely the “so-called sidereal astronomy, which to-day constitutes the only grave scientific aberration peculiar to celestial studies.”[128] Ten years later, in the first volume of the Politique positive, he “regenerates” astronomy from the synthetic point of view. He is no longer content to limit it to the knowledge of the solar system. He confines the particular study of our world within narrow limits. Astronomy, like the other sciences, from objective must become subjective. Instead of the vague (that is to say indefinite) study of the heavens its end must be the knowledge of the earth, and the consideration of the other celestial bodies only in their relation to the human planet. At this price alone can the unity of this science be secured.[129]
Thus Comte came back to Aristotle’s closed world with the earth as its centre. He points it out himself in showing in what way he differs from the ancient conception. “This unity,” he says, “existed for the ancients, but was of an absolute character which at that time was legitimate.” When the motion of our planet became known, the ancient constitution of celestial science might merely have been modified “by preserving in it, as subjective, the centre which was at first supposed to be objective.” That would have sufficed to change astronomy from an absolute science to a relative one. Undoubtedly the ancients were deceived in believing the earth to be the centre of the world; but, in order to correct their error, it sufficed to say, the centre of our world. The subjective synthesis “indeed concentrates the celestial studies round the earth.” The other stars only deserve our attention in so far as the knowledge of our planet requires it. Comte ends by saying in the fourth volume of the Politique positive that, strictly speaking, the study of the sun and moon would suffice. We may add to them the ancient planets, but not the “little telescopic planets.”[130]
This progressive narrowing of the astronomical domain does not indicate a radical change in Comte’s philosophical thought. It only results from the growing subordination of the scientific interest to other superior interests. To know for the sake of knowing, appears to Comte to be a wrong use of the human intellect. The Newtons and the Laplaces in the past have fulfilled a necessary function, and humanity owes them eternal gratitude. They struck a decisive blow against theological and metaphysical philosophy; and secured the victory for the positive spirit. In their time scientific speculation which tended to the discovery of the laws of phenomena, and especially of celestial phenomena, was at once the most sublime and the most useful occupation which those men of genius could set themselves. But now that their efforts have culminated in the foundation of positive philosophy, and this philosophy itself in the “final religion,” there is no longer any reason to continue researches with which henceforth humanity can dispense. We must even “cut down many idle acquisitions.”[131] In a word, from the religious point of view, Comte, in order to remedy the anarchy of science, suppresses its liberty.