But still the main and most diverting Point of the Enquiry is behind, which is the placing some Spectators in these new Discoveries, to enjoy these Creatures we have planted them with, and to admire their Beauty and Variety. And among all, that have never so slightly meddled with these matters, I don’t find any that have scrupled to allow them their Inhabitants: not Men perhaps like ours, but  some Creatures or other endued with Reason. For all this Furniture and Beauty the Planets are stock’d with seem to have been made in vain, without any design or end, unless there were some in them that might at the same time enjoy the Fruits, and adore the wise Creator of them. But this alone would be no prevailing Argument with me to allow them such Creatures. For what if we should say, that God made them for no other design, but that he himself might see (not as we do ’tis true; but that he that made the Eye sees, who can doubt?) and delight himself in the contemplation of them? For was not Man himself, and all that the whole World contains, made upon this very account? That which makes me of this opinion, that those Worlds are not without such a Creature endued with Reason, is, that otherwise our Earth would have too much the advantage of them, in being the only part of the Universe that could boast of such a Creature so far above, not only Plants and Trees, but all  Animals whatsoever: a Creature that has a Divine somewhat within him, that knows, and understands, and remembers such an innumerable number of things; that deliberates, weighs and judges of the Truth: a Creature upon whose account, and for whose use, whatsoever the Earth brings forth seems to be provided. For every thing here he converts to his own ends. With the Trees, Stones, and Metals, he builds himself Houses: the Birds and Fishes he sustains himself with: and the Water and Winds he makes subservient to his Navigation; as he doth the sweet Smell and glorious Colours of the Flowers to his Delight. What can there be in the Planets that can make up for its Defects in the want of so noble an Animal? If we should allow Jupiter a greater variety of other Creatures, more Trees, Herbs and Metals, all these would not advantage or dignify that Planet so much as that one Animal doth ours by the admirable Productions of his penetrating Wit. If I am out in this, I do not know when  to trust my Reason, and must allow my self to be but a poor Judg in the true estimate of things. C. Huygens, Cosmotheoros, “Rational Animals in the Planets”.
I have mentioned before (recall here) that in an unpublished manuscript (from 1691) Huygens embraced what one may call an ‘aesthetic-maximazation-conservation principle.’ This principle asserts that no natural beauty is wasted and so requires spectators to enjoy it. Because the night-skies on other planets are lovely there must be extraterrestrials to enjoy it (Newton, too, embraces this implication). Huygens's God would not waste the opportunity to have sentient beings enjoy the aesthetic pleasure. Neither God nor nature does anything in vain, after all. " I had noted that Huygens mentions the principle in the (posthumously published) Cosmotheoros, but rejects it as decisive (“this alone would be no prevailing Argument with me to allow them such Creatures.”)
It’s not entirely clear why Huygens rejects it the ‘aesthetic-maximazation-conservation principle’ as offering us grounds to infer extraterrestrials. Perhaps, it’s because he comes to see that there are wasted aesthetic opportunities in the universe: while he is firm that our Sun’s planets have inhabitants, and that this is so of planets of other Suns throughout the universe, he has decided that moons (and Suns) do not have inhabitants.
Another possible ground for rejecting the ‘aesthetic-maximazation-conservation principle’ is that it entails, as Huygens notes, that we humans may exist for God’s aesthetic pleasure (“God made them for no other design, but that he himself might see…and delight himself in the contemplation of them?”) I believe Huygens’s suggestion to this effect is a rhetorical question (even if Huygens embraces providential deism in the Cosmotheoros.)) A reason for thinking this is a rhetorical question is that in the quoted passage, Huygens does not consider this answer – we exist for God’s aesthetic pleasure – as decisive, not to mention that this position would bring Huygens uncomfortably close to Epicureanism --a position he rejects explicitly in the Cosmotheoros.
Rather, Huygens embraces a more Copernican, ‘all Solar Systems are Created Equal,’ principle. A more modest version of the principle is used within solar systems: ‘all planets are Created Equal’ principle. Of course, Huygens recognizes that gravity and heat will be different in different planets, and that the night sky will offer different opportunities to develop one’s astronomy. In fact, part of the fun of the book is having Huygens imagine for his reader what experiencing the galaxy would be like for extraterrestrials. Here’s a nice example:
if Amazement and Fear at the Eclipses of the Moon and Sun gave the first occasion to the study of Astronomy [on Earth], as they say it did, then it’s almost impossible that Jupiter and Saturn should be without it; the Argument being of much greater force in them, by reason of the; daily Eclipses of their Moons, and the frequent ones of the Sun to their Inhabitants. So that if a Person disinterested in his Judgment, and equally ignorant of the Affairs of all the Planets, were to give his Opinion in the matter, I don’t doubt he would give the cause for Astronomy to those two Planets rather than us.
It is notable that Huygens here draws on (Epicurean) tropes which became central to Enlightenment thought (in fact, the parallels with Adam Smith's History of Astronomy are uncanny):+ ancient science is grounded in the unpleasant experience of the spectacle the heavens offer us. The early inhabitants of the outer planets have more regular reasons – and better potential data! -- to begin to track their heavens. An implication that Huygens leaves hanging is that some extraterrestrials will surpass us in scientific knowledge. (Later, when he explicitly gets to the issue, he goes no further than to say that: “there’s no reason to think the Planetarians less ingenious or happy than our selves; we have gained our point, and ’tis probable that they are as skilful Astronomers as we can pretend to be.”)
On the whole Huygens’s reasons for suggesting his ‘all Solar Systems are Created Equal,’ principle are not very interesting (or persuasive--they rely on analogies). But after driving home the vast distances of the visible universe, and reminding his readers of the even vaster distances of the (infinite) universe invisible to us, he introduces a striking, and I think consequential, thought experiment, near the end of his book that helps explain why he adopts this principle:
For then why may not every one of these Stars or Suns have as great a Retinue as our Sun, of Planets, with their Moons, to wait upon them? Nay there’s a manifest reason why they should. For let us fancy our selves placed at an equal distance from the Sun and fix’d Stars; we should then perceive no difference between them. For, as for all the Planets that we now see attend the Sun, we should not have the least glimpse of them, either that their Light would be too weak to affect us, or that all the Orbs in which they move would make up one lucid point with the Sun. In this station we should have no occasion to imagine any difference between the Stars, and should make no doubt if we had but the sight, and knew the nature of one of them, to make that the Standard of all the rest. We are then plac’d near one of them, namely, our Sun, and so near as to discover six other Globes moving round him, some of them having others performing them the same Office. Why then shall not we make use of the same Judgment that we would in that case; and conclude, that our Star has no better attendance than the others? So that what we allow’d the Planets, upon the account of our enjoying it, we must likewise grant to all those Planets that surround that prodigious number of Suns. They must have their Plants and Animals, nay and their rational ones too, and those as great Admirers, and as diligent Observers of the Heavens as our selves; and must consequently enjoy whatsoever is subservient to, and requisit for such Knowledge. (Italics added.)
There are three underlying intuitions here: first, on a cosmic scale, all Suns are equal with no marks to distinguish them and so no marks to elevate them over others. Second, most planets, including ours, are invisible at most points in the universe, and so have no claim to privileged status. Third, there is no rational ground to make one planet (ours) the measure of others. Man is not the measure of things once we see ourselves in our proper place. The nod to Protagoras is important.
It is, by now, a familiar claim that Copernicanism decenters humans and our solar system. We are not at the center of creation anymore. But even so various species of humanism have survived, even thrived in the ages of Copernicanism. (Huygens knew this up close; his father was one of the great Renaissance men of his age.) Throughout the Cosmotheoros, Huygens hesitates to grant that we humans are at the pinnacle of earthly life. He is especially impressed by birds (challenging Descartes's treatment of them as mere mechanical entities):
Birds, liv’d with more pleasure and happiness than Man could with all his Wisdom. For they have as great a gusto of bodily Pleasures as we, let the new Philosophers say what they will, who would have them go for nothing but Clocks and Engines of Flesh; a thing which Beasts so plainly confute by crying and running away from a stick, and all other actions, that I wonder how anyone could subscribe to so absurd and cruel an Opinion. Nay I can scarce doubt but that Birds feel no small pleasure in their easy, smooth sailing through the Air; and would much more if they but knew the advantages it hath above our slow and laborious Progression.*
Huygens notes that from the impartial and disinterested perspective of the universe [as it were, sub species universalis], our planet is just a “small speck of dirt."* That is, Huygens thinks embracing the 'Solar Systems are Created Equal,’ principle also has obvious implications for "Wisdom and Morality," but about that some other time more.