The Plurality of Worlds

by William Whewell

23. It is true, it has been conjectured, by high authority,[20] that this accelerated rapidity of the periods of Algol will not continue; but will gradually relax, and then be changed to an increase; like many other cyclical combinations in astronomy. But this conjecture seems to have little to support it. The cases in which an acceleration of motion is retarded, checked, and restored, all belong to our Solar System; and to assume that Algol, like the solar system, has assumed a permanent and balanced condition, is to take for granted precisely the point in question. We know of no such cycles among the fixed stars, at least with any certainty; for the cycle proposed for Mira must be considered as greatly needing confirmation; considering how long is the cycle, and how recent the suggestion of its existence.

24. And even in the solar system, we have accelerated motions, in which no mathematician or astronomer looks for a check or regress of the acceleration. No one expects that Encke's comet will cease to be accelerated, and to revolve in periods continually shorter; though all the other motions hitherto observed in the system are cyclical. In the case of a fixed star, we have much less reason to look for such a cycle, than we have in Encke's comet. But further: with regard to the existence of such a cycle of faster and slower motion in the case of Algol, the most recent observed facts are strongly against it; for it has been observed by Argelander, that not only there is a diminution of the period, but that this diminution proceeds with accelerated rapidity; a course of events which, in no instance, in the whole of the cosmical movements, ends in a regression, retardation, and restoration of the former rate. We are led to believe, therefore, that this remarkable luminary will go on revolving faster and faster, till its extreme point of condensation is attained. And in the meantime, we have very strong reasons to believe that this mutable body is not, like the sun, a permanent centre of a permanent system; and that any argument drawn from its supposed likeness to the sun, in favor of the supposition that the regions which are near it are the seats of habitation, is quite baseless.

25. There are other phenomena of the Fixed Stars, and other conjectures of astronomers respecting them, which I need not notice, as they do not appear to have any bearing upon our subject. Such are the "proper motions" of the stars, and the explanation which has been suggested of some of them; that they arise from the stars revolving round other stars which are dark, and therefore invisible. Such again is the attempt to show that the Sun, carrying with it the whole Solar System, is in motion; and the further attempt to show the direction of this motion; and again, the hypothesis that the Sun itself revolves round some distant body in space. These minute inquiries and bold conjectures, as to the movements of the masses of matter which occupy the universe, do not throw any light on the question whether any part besides the earth is inhabited; any more than the investigation of the movements of the ocean, and of their laws, could prove or disprove the existence of marine plants and animals. They do not on that account cease to be important and interesting subjects of speculation; but they do not belong to our subject.

26. In Fontenelle's Dialogues on the Plurality of Worlds, a work which may be considered as having given this subject a place in popular literature, he illustrates his argument by a comparison, which it may be worth while to look at for a moment. The speaker who asserts that the moon, the planets, and the stars, are the seats of habitation, describes the person, who denies this, as resembling a citizen of Paris, who, seeing from the towers of Notre Dame the town of Saint Denis, (it being supposed that no communication between the two places had ever occurred,) denies that it is inhabited, because he cannot see the inhabitants. Of course the conclusion is easy, if we may thus take for granted that what he sees is a town. But we may modify this image, so as to represent our argument more fairly. Let it be supposed that we inhabit an island, from which innumerable other islands are visible; but the art of navigation being quite unknown, we are ignorant whether any of them are inhabited. In some of these islands, are seen masses more or less resembling churches; and some of our neighbors assert that these are churches; that churches must be surrounded by houses; and that houses must have inhabitants. Others hold that the seeming churches are only peculiar forms of rocks. In this state of the debate, everything depends upon the degree of resemblance to churches which the forms exhibit. But suppose that telescopes are invented, and employed with diligence upon the questionable shapes. In a long course of careful and skilful examination, no house is seen, and the rocks do not at all become more like churches, rather the contrary. So far, it would seem, the probability of inhabitants in the islands is lessened. But there are other reasons brought into view. Our island is a long extinct volcano, with a tranquil and fertile soil; but the other islands are apparently somewhat different. Some of them are active volcanoes, the volcanic operations covering, so far as we can discern, the whole island; others undergo changes, such as weather or earthquakes may produce; but in none of them can we discover such changes as show the hand of man. For these islands, it would seem the probability of inhabitants is further lessened. And so long as we have no better materials than these for forming a judgment, it would, surely, be accounted rash, to assert that the islands in general are inhabited; and unreasonable, to blame those who deny or doubt it. Nor would such blame be justified by adducing theological or à priori arguments; as, that the analogy of island with island makes the assumption allowable; or that it is inconsistent with the plan of the Creator of islands to leave them uninhabited. For we know that many islands are, or were long, uninhabited. And if ours were an island occupied by a numerous, well-governed, moral, and religious race, of which the history was known, and of which the relation to the Creator was connected with its history; the assumption of a history, more or less similar to ours, for the inhabitants of the other islands, whose existence was utterly unproved, would, probably, be generally deemed a fitter field for the romance-writer than for the philosopher. It could not, at best, rise above the region of vague conjecture.

27. Fontenelle, in the agreeable book just referred to, says, very truly, that the formula by which his view is urged on adversaries is, Pourquoi non? which he holds to be a powerful figure of logic. It is, however, a figure which has this peculiarity, that it may, in most cases, be used with equal force on either side. When we are asked Why the Moon, Mercury, Saturn, the system of Sirius, should not be inhabited by intelligent beings; we may ask, Why the earth in the ages previous to man might not be so inhabited? The answer would be, that we have proof how it was inhabited. And as to the fact in the other case, I shall shortly attempt to give proof that the Moon is certainly not, and Mercury and Saturn probably not inhabited. With regard to the Fixed Stars, it is more difficult to reason; because we have the means of knowing so little of their structure. But in this case also, we might easily ask on our side, Pourquoi non? Why should not the Solar System be the chief and most complete system in the universe, and the Earth the principal planet in that System? So far as we yet know, the Sun is the largest Sun among the stars; and we shall attempt to show, that the Earth is the largest solid opaque globe in the solar system. Some System must be the largest and most finished of all; why not ours? Some planet must be the largest planet; why not the Earth?

28. It should be recollected that there must be some system which is the most complete of all systems, some planet which is the largest of all planets. And if that largest planet, in the most complete system, be, after being for ages tenanted by irrational creatures, at last, and alone of all, occupied by a rational race, that race must necessarily have the power of asking such questions as these: Why they should be alone rational? Why their planet should be alone thus favored? If the case be ours, we may hope to be then able to answer these questions, when we can explain the most certain fact which they involve; Why the Earth was occupied so long by irrational creatures, before the rational race was placed upon it? The mere power of asking such questions can prove or disprove nothing; for it is a power which must equally subsist, whether the human inhabitants of the earth be or be not the only rational population which the universe contains. If there be a race thus favored by the Creator, they must, at that stage of their knowledge in which man now is, be able to doubt, as man does, of the extent and greatness of the privilege which they enjoy.

29. The argument that the Fixed Stars are like the Sun, and therefore the centres of inhabited systems as the Sun is, is sometimes called an argument from Analogy; and this word Analogy is urged, as giving great force to the reasoning. But it must be recollected, that precisely the point in question is, whether there is an analogy. The stars, it is said, are like the Sun. In what respects? We know of none, except in being self-luminous; and this they have in common with the nebulæ, which, as we have seen, are not centres of inhabited systems. Nor does this quality of being self-luminous at all determine the degree of condensation of a star. Sirius may be less than a hundredth or a thousandth of the density of the Sun. But the Stars, it may be further urged, are like the Sun in turning on their axes. To this we reply, that we know this only of those stars in which, the very phenomenon which proves their revolution, proves also that they are unlike the Sun, in having one side darker than the other. Add to which, their revolution is not connected with the existence of planets, still less of inhabitants of planets, in any intelligible manner. The resemblance, therefore, so far as it bears upon the question, is confined to one single point, in the highest degree ambiguous and inconclusive; and any argument drawn from this one point of resemblance, has little claim to be termed an argument from analogy.[21]

30. On a subject on which we know so little, it is difficult to present any view which deserves to be regarded as an analogy. We see, among the stars, nebulæ more or less condensed, which are possibly, in some cases, stages of a connected progress towards a definite star; and it may be, to a star with planets in permanent orbits. We see, in our planet, evidence of successive stages of a connected series of brute animals, preceded perhaps by various stages of lifeless chaos. If the histories of the Sun, and of all the stars, are governed by a common analogy, the nebulous condensation, and the stages of animal life, may be parts of the same continued series of events; and different stars may be at different points of that series. But even on this supposition, but a few of the stars may be the seats of conscious life, and none, of intelligence. For among the stars which have condensed to a permanent form, how many have failed in throwing off a permanent planet! How many may be in some stage of lifeless chaos! We must needs suppose a vast number of stages between a nebular chaos and the lowest forms of conscious life. Perhaps as many as there are fixed stars; and far more than there are of stars which become fertile of life: so that no two systems may be at the same stage of the planetary progress. And if this be so,—our system being so complicated, that we must suppose it peculiarly developed, having the largest Sun that we know of, and our Earth being (as we shall hereafter attempt to prove) the largest solid planet that we know of,—this Earth may be the sole seat of the highest stage of planetary development.

31. The assumption that there is anything of the nature of a regular law or order of progress from nebular matter to conscious life,—a law which extends to all the stars, or to many of them,—is in the highest degree precarious and unsupported; but since it is sometimes employed in such speculations as we are pursuing, we may make a remark or two connected with it. If we suppose, on the planets of other systems, a progress in some degree analogous to that which geology shows to have occurred on the Earth, there may be, in those planets, creatures in some way analogous to our vegetables and animals; but analogy also requires that they should differ far more from the terrestrial vegetables and animals of any epoch, than those of one epoch do from those of another; since they belong to a different stellar system, and probably exist under very different conditions from any that ever prevailed on the Earth. We are forbidden, therefore, by analogy, to suppose that on any other planet there was such an anatomical progression towards the form of man, as we can discern (according to some eminent physiologists) among the tribes which have occupied the Earth. Are we to conceive that the creatures on the planets of other systems are, like the most perfect terrestrial animals, symmetrical as to right and left, vertebrate, with fore limbs and hind limbs, heads, organs of sense in their heads, and the like? Every one can see how rash and fanciful it would be to make such suppositions. Those who have, in the play of their invention, imagined inhabitants of other planets, have tried to avoid this servile imitation of terrestrial forms. Here is Sir Humphry Davy's account of the inhabitants of Saturn. "I saw moving on the surface below me, immense masses, the forms of which I find it impossible to describe. They had systems for locomotion similar to that of the morse or sea-horse, but I saw with great surprise that they moved from place to place by six extremely thin membranes, which they used as wings. I saw numerous convolutions of tubes, more analogous to the trunk of the elephant, than to anything else I can imagine, occupying what I supposed to be the upper parts of the body."[22] The attendant Genius informs the narrator, that though these creatures look like zoophytes, they have a sphere of sensibility and intellectual enjoyment far superior to that of the inhabitants of the Earth. If we were to reason upon a work of fancy like this, we might say, that it was just as easy to ascribe superior sensibility and intelligence to zoophyte-formed creatures upon the Earth, as in Saturn. Even fancy cannot aid us in giving consistent form to the inhabitants of other planets.

32. But even if we could assent to the opinion, as probable, that there may occur, on some other planet, progressions of organized forms analogous in some way to that series of animal forms which has appeared upon the earth, we should still have no ground to assume that this series must terminate in a rational and intelligent creature like man. For the introduction of reason and intelligence upon the Earth is no part nor consequence of the series of animal forms. It is a fact of an entirely new kind. The transition from brute to man does not come within the analogy of the transition from brute to brute. The thread of analogy, even if it could lead us so far, would break here. We may conceive analogues to other animals, but we could have no analogue to man, except man. Man is not merely a higher kind of animal; he is a creature of a superior order, participating in the attributes of a higher nature; as we have already said, and as we hope hereafter further to show. Even, therefore, if we were to assume the general analogy of the Stars and of the Sun, and were to join to that the information which geology gives us of the history of our own planet; though we might, on this precarious path, be led to think of other planets as peopled with unimagined monsters; we should still find a chasm in our reasoning, if we tried, in this way, to find intelligent and rational creatures in planets which may revolve round Sirius or Arcturus.