By the end of the 17th century, attacks on the Cartesian philosophy were often rather unoriginal and derivative, philosophically speaking. Yet many continued to reflect new preoccupations of philosophy in the post-Cartesian period in imaginative ways. A good example of this is the Jesuit Gabriel Daniel's 1690 satire, Voiage du Monde de Descartes, which envisions an interstellar journey through Descartes's World, now conceived not as a mere thought experiment, but as a sort of science-fiction construction, as a parallel possible world. Such a construal, as I would like to argue, though not so significant with respect to the history of critiques of Cartesianism, nonetheless reveals important developments in the deployment of the philosophical concept of 'world'.
It will be useful perhaps to begin with a question that might seem too obvious to need asking, but that in the 17th century was at the center of a number of fundamental philosophical debates: what, in general, is a world?
It is a commonplace in history-of-science scholarship since Alexandre Koyré to acknowledge that the beginning of the modern period witnessed a transition between two very different cosmological models, from the closed world to the infinite universe, as the title of Koyré’s most influential book put it. But beyond its closedness, there are a few other important features to note in the most familiar cosmological models inherited from antiquity.
Aristotle, like many of his contemporaries, had found it useful to divide the world into two basic regions: the superlunar and the sublunar. The first of these is the home of the celestial bodies. They are immortal and, relatedly, unmixed. They are entirely composed of a single element, and therefore cannot be corrupted, cannot cease to be, through a separating out of their constituents. They must of necessity move, since only the unmoved mover is free of all change, but their motion duplicates the perfect quiescence of the supreme being as much as possible: it is perfectly circular, and so the immortal celestial bodies always come back to the very points in the cosmos that their motion has already traced. Sublunar bodies, for their part, consist in various mixtures of earth, air, fire, and water, and for this reason are destined sooner or later to come apart, to cease to be, when the elements that constitute them go their separate ways. Some sublunar beings, which we today would call ‘biological’, in turn imitate the circular motion of the celestial spheres --in much the same way their circular motion imitates the stillness of the unmoved mover-- through what Aristotle calls ‘cyclical’ motion (as distinct from circular), which is, as he explains, a ‘cycling back upon oneself’ in sexual reproduction, which wins for the individual mortal natural being a share of eternity ‘in kind if not in number’.
So much for the sublunar and the superlunar. What about the moon itself? What side is it on? If it is on neither, then what is the significance of such a fundamental ontological divide? In this paper I would like to look at a few ways in which the moon, as a boundary entity of crucial significance, has since antiquity played a fundamental role in thought experiments that have helped natural philosophers to come to a picture of nature as a whole, to arrive at least partially at what Thomas Nagel would call a ‘view from nowhere’. This is particularly clear in the early modern period when the Aristotelian two-region picture of the cosmos is rejected in favor of a picture of the cosmos on which the same laws hold everywhere in the same way, and the study of projectiles on the earth’s surface may be carried out in exactly the same way as the study of planetary orbits.
In On the Generation of Animals, Aristotle makes a special plea, evidently intended for unnamed critics, for the idea that there is something worthwhile about the study of perishable living creatures. He acknowledges that the celestial bodies are more divine, in large part because they are immortal (and also, by the way, intelligent), but, he adds, they are also very far away, and therefore hard to study. Living creatures are less divine, but they are also close at hand. It would be a mistake, moreover, to exclude them from the realm of the divine entirely simply in view of their perishability, for here too, as Aristotle says, there is something beautiful and wonderful. He cites in this connection the fragment of Heraclitus who, when caught lounging naked on a stove by distinguished visitors, protests: “Here too dwell gods.” Here, what Aristotle is first and foremost doing, in his way, is defending the integrity and the legitimacy of the study of living beings.
‘The world’, for Aristotle, is a system in which certain events happen in one region, which in turn trickle down and cause homologous yet different events in another region. Astronomy and ‘biology’ (to use a blatant anachronism) are two domains of a unified science of nature. In his own, directly inverse, way, Descartes too would aim to present biology as a regional instance of a general science of nature: this aim is precisely what is at stake in Descartes’s expressed desire to explain the formation of the fetus “in the same manner as the rest,” that is, by appeal to the same minor laws that also explain the motion of projectiles and the orbit of planets. Here, plainly, the unity of the superlunar and the sublunar is no longer a result of some cosmic trickle-down from the former to the latter, but rather of the fact that for Descartes no region is special, no region has causal or explanatory priority over any other.
Aristotle’s world is closed, as Koyré emphasized, but it is also hierarchically structured. In the modern period, we do indeed witness a sudden and tremendous expansion of the world, an ‘infinitization’, but also a destructuring or disassembly. And here, there were different available models for reconceiving the structure of the cosmos. The most significant division, perhaps, was between those who imagined the new infinite world as, so to speak, an ‘infinite extension of the same’, and those who envisioned an infinite fracturing or reduplication. That is, one could go with Descartes, and maintain that the world is simply extensus indefinite, that it consists in res extensa, in the same way it does right here, however far out from here one may travel; or one could take up the alternative view, espoused by Henry More after his initial exposure to the philosophy of Descartes, according to which there are infinite worlds, or infinite centers of well-structured kosmoi. Thus More reflects in his Democritus Platonissans of 1646 on the ‘fair glistering lights in heaven’, that:
If onely for this world they were intended
Nature would have adorn'd this azure Round
With better Art.
The evident disorder of the heavens is for More a reason to suppose that each star is the center of its own world, rather than having its raison d’être in relation to the earth, or indeed to life on earth.
There is, in the multiplication of worlds from More to Bernard de Fontenelle, who published his famous Conversations on the Plurality of Worlds in 1686, a deep ambiguity in the meaning of ‘world’. Are we to understand by a ‘many-worlds’ hypothesis that there are multiple, inaccessible, self-contained ‘realities’, so to speak; or are we rather to understand that there are simply multiple regional centers of one and the same reality? A clear illustration of this ambiguity (which extends in important senses even into David Lewis’s defense of modal realism) may be found, for example, in the Spanish-Inca political philosopher Garcilaso de la Vega’s Royal Commentaries of the Incas, first published in 1609, in which the author devotes the first chapter of the first book to denying the impious view that there are ‘many worlds’, and affirming that the ‘New World’ is so called only because it was discovered recently, not because it is in any sense a discrete or independent reality. “If there are any men who imagine that there are many worlds,” de la Vega writes, “there is no other response to offer them, unless they persist in their heretical belief until they are disabused of it in hell.”
We see in de la Vega that the problem of many worlds extended all the way from the cosmographical to the much more fine-grained questions of geography, and the Spanish-Inca author is firm in his objection to what might be called, in deference to Lewis, ‘many-worlds realism’. For him, the term ‘worlds’ offers nothing more than a poetic way of describing spatially distant but potentially traversable regions. We might say of Henry More that he takes just the opposite approach: for him, spatial distance, or at least a significant amount of spatial distance, is already sufficient for establishing the true, proper distinctness of one ‘world’ in relation to another. What is more, he even manages to extend his ‘many worlds’ realism to Descartes by eliding his own conception of ‘world’ with the French philosopher’s model of the world as consisting in infinite extension: “Nay,” More writes, “and that sublime and subtil Mechanick too, Des Chartes, though he seem to mince it must hold infinitude of worlds, or which is as harsh one infinite one. For what is his mundus indefinite extensus, but extensus infinite? Else it sounds onely infinitus quoad nos but simpliciter finitus.” Even this early, we see More’s characteristic attitude towards Descartes already in place: to the extent that More can claim common cause with Descartes, it is only by separating the latter’s claims from the concerns that motivated them. Prima facie, it is an odd thing to recruit Descartes for one’s case for the infinity of worlds, and the case for these worlds is not much strengthened by Descartes’s commitment to one indefinitely extended world.
Willy-nilly, then, by entering into the world business, Descartes becomes implicated in the problem of worlds in the plural. This implication continues long after More, and can perhaps be better understood by a brief consideration of Fontenelle’s famous work on the plurality of worlds. The moon is a world like ours," Fontenelle causes his protagonist to assert, "and to all appearance, inhabited." In the Conversations, Fontenelle is evidently playing on the dual signification of the French notion of ‘monde’, as describing both the physical universe as well as human society, or, more particularly, a single, self-contained human society. Where there are intelligent creatures living together and constituting un monde for one another, it follows that this amounts to a distinct world. But if a part of the cosmos is made out of the same matter as the earth is, and is subject to the same laws, then it naturally follows that it will be inhabited by intelligent beings like us. There are multiple worlds not in any strong Lewisian sense of parallel intraversable realities, but simply in the sense that there are multiple mondes with, ordinarily, no traffic between them, even if in principle one could in fact move from one to another. And the existence of such multiple mondes is a direct and inevitable consequence of the fact that the entire cosmos is governed by the same basic laws, and can be explained, like the formation of the earthling fetus, “in the same manner as the rest.” The dismantling of the structured hierarchy of the ancients leads directly to extraterrestrials.
Or at least this is the conclusion to which many late 17th-century satirists and fantasists were drawn. There appears in fact to be a direct translation of the Cartesian doctrine of explanation “in the same manner as the rest,” into what would come to be known as ‘the Harlequin principle’: toujours et partout, c’est tout comme ici [‘always and everywhere, it’s the same as it is here’]. This is the catch-phrase exclaimed by a chorus of lunar characters in Anne Mauduit Nolant de Fatouville’s comic opera, Arlequin, empereur dans la Lune, spoken in unison as a response to reports about the hypocrisy and vanity of social life on earth. In other words, wherever there is a monde, the same human (or humanoid) comedy will repeat itself. Later, in the New Essays concerning Human Understanding of 1704, G. W. Leibniz would adapt this catch-phrase as a pithy summary of his own theory of monads, according to which the world consists in an infinity of ultimately identical perceiving substances, which are individuated only in virtue of their perspective from within the order of reciprocal perception. From the hierarchically structured and closed world, we move through the world as indefinite extension of the same, a view crystallized in the formula partout comme ici, which is then taken up as the motto of a metaphysical system grounded in the infinite repetition of perception, fractured, kaleidoscopically, into infinitely many points of view.
Leibniz does not mean to present his system of monads as science-fiction. He takes it as a true account of how the world really is, and this account, while seldom taken as true today, is considered worthy of study by the designers of undergraduate curricula in philosophy. And yet it is, as we have begun to discern here, in large part a sort of following-out of the implications of a way of thinking about the world, or about the plurality of worlds, that was already set in motion by Descartes. As Leibniz himself acknowledges, important, if theoretically unsophisticated, voice was given to this way of thinking not only by philosophers, but also by fantasists such as Nolant de Fatouville (elsewhere in the New Essays, Leibniz also mentions Cyrano de Bergerac in the same connection). It is perhaps worth taking more seriously the role of science fiction, and of literary and satirical works more generally, in the uses to which the concept of 'world' was put throughout the course of early modern philosophy.