The first thought that men had concerning the heavenly bodies was anobvious one: they were lights. There was a greater light to rule the day;a lesser light to rule the night; and there were the stars also.
In those days there seemed an immense difference between the earth uponwhich men stood, and the bright objects that shone down upon it from theheavens above. The earth seemed to be vast, dark, and motionless; thecelestial lights seemed to be small, and moved, and shone. The earth wasthen regarded as the fixed centre of the universe, but the Copernicantheory has since deprived it of this pride of place. Yet from anotherpoint of view the new conception of its position involves a promotion,since the earth itself is now regarded as a heavenly body of the sameorder as some of those which shine down upon us. It is amongst them, andit too moves and shines--shines, as some of them do, by reflecting thelight of the sun. Could we transport ourselves to a neighbouring world,the earth would seem a star, not distinguishable in kind from the rest.
But as men realized this, they began to ask: "Since this world from adistant standpoint must appear as a star, would not a star, if we couldget near enough to it, show itself also as a world? This world teems withlife; above all, it is the home of human life. Men and women, gifted withfeeling, intelligence, and character, look upward from its surface andwatch the shining members of the heavenly host. Are none of these the homeof beings gifted with like powers, who watch in their turn the movementsof that shining point which is our world?"
This is the meaning of the controversy on the Plurality of Worlds whichexcited so much interest some sixty years ago, and has been with us moreor less ever since. It is the desire to recognize the presence in the orbsaround us of beings like ourselves, possessed of personality andintelligence, lodged in an organic body.
This is what is meant when we speak of a world being "inhabited." It wouldnot, for example, at all content us if we could ascertain that Jupiter wascovered by a shoreless ocean, rich in every variety of fish; or that thehard rocks of the Moon were delicately veiled by lichens. Just as norichness of vegetation and no fulness and complexity of animal life wouldjustify an explorer in describing some land that he had discovered asbeing "inhabited" if no men were there, so we cannot rightly speak of anyother world as being "inhabited" if it is not the home of intelligentlife. If the life did not rise above the level of algæ or oysters, theglobe on which they flourish would be uninhabited in our estimation, andits chief interest would lie in the possibility that in the course of ageslife might change its forms and develop hereafter into manifestations withwhich we could claim a nearer kinship.
On the other hand, of necessity we are precluded from extending ourenquiry to the case of disembodied intelligences, if such be conceivedpossible. All created existences must be conditioned, but if we have noknowledge of what those conditions may be, or means for attaining suchknowledge, we cannot discuss them. Nothing can be affirmed, nothingdenied, concerning the possibility of intelligences existing on the Moonor even in the Sun if we are unable to ascertain under what limitationsthose particular intelligences subsist. Gnomes, sylphs, elves, andfairies, and all similar conceptions, escape the possibility of discussionby our ignorance of their properties. As nothing can be asserted of themthey remain beyond investigation, as they are beyond sight and touch.
The only beings, then, the presence of which would justify us in regardinganother world as "inhabited" are such as would justify us in applying thatterm to a part of our own world. They must possess intelligence andconsciousness on the one hand; on the other, they must likewise havecorporeal form. True, the form might be imagined as different from that wepossess; but, as with ourselves, the intelligent spirit must be lodged inand expressed by a living material body. Our enquiry is thus rendered aphysical one; it is the necessities of the living body that must guide usin it; a world unsuited for living organisms is not, in the sense of thisenquiry, a "habitable" world.
The discussion, as it was carried on sixty years ago by Dr. Whewell andSir David Brewster, was essentially a metaphysical, almost a theologicalone, and it was chiefly considered in its supposed relationship to certainreligious conceptions. It was urged that it was derogatory to the wisdomand goodness of the Creator to suppose that He would have created so manygreat and glorious orbs without having a definite purpose in so doing, andthat the only purpose for which a world could be made was that it might beinhabited. So, again, when Dr. A. R. Wallace revived the discussion in1903, he clearly had a theological purpose in his opening paper, though hewas taking the opposite view from that held by Brewster half a centuryearlier.
For myself, if there be any theological significance attaching to thesolving of this problem, I do not know what it is. If we decide that thereare very many inhabited worlds, or that there are few, or that there isbut one--our own--I fail to see how it should modify our religiousbeliefs. For example: explorers have made their way across the Antarcticcontinent to the South Pole but have found no "inhabitant" there. Has thisfact any theological bearing? or if, on the contrary, a race of men hadbeen discovered there, what change would it have made in the theologicalposition of anyone? And if this be so with regard to a new continent onthis earth, why should it be different with regard to the continents ofanother planet?
The problem therefore seems not to be theological or metaphysical, butpurely physical. We have simply to ask with regard to each heavenly bodywhich we pass in review: "Are its physical conditions, so far as we canascertain them, such as would render the maintenance of life possible uponit?" The question is not at all as to how life is generated on a world,but as to whether, if once in action on a particular world, its activitiescould be carried on.