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At first glance, few groups of thinkers seem as antithetical and adverse to philosophical scepticism as the atheists of the late Enlightenment: Denis Diderot, the baron d’Holbach, and their close friend and collaborator, and fellow devotee of Holbach’s salon, Jacques-André Naigeon. They all diversely wrote, at various times, that we knew that the world we observed arose from matter in motion according to fixed, knowable laws of nature. They were all determinists. They all argued, diversely, that the achievements of recent physics and the life sciences had given us models for understanding both nature and human phenomena as a part of nature. In some essential ways, they all could be utilized far more easily as part of a history of positivism than of a history of scepticism.

As I have argued elsewhere, they all indeed addressed and noted “la faiblesse de l’esprit humain [the weakness of the human mind]” because, among other things, they had to explain and account for what for them was the quite remarkable human penchant for what they saw as irrational theistic belief and superstition.Footnote 1 It is not clear, however, that a belief in the weakness of the human mind should be taken as a serious engagement with philosophical scepticism. Let us look more closely.

Indeed, few rejections of formal scepticism are more categorical than that of Diderot in the first Entretien with d’Alembert in Diderot’s Rêve de d’Alembert. Although neither d’Alembert, Bordeu, or Julie de L’Espinasse would have recognized themselves in Diderot’s portraits (he must have had a grand time writing it, especially given d’Alembert’s actual caution concerning scandal). D’Alembert, in his metaphysics at least, was, in fact, something of a genuine philosophical sceptic. Diderot and d’Alembert, in Diderot’s account, are discussing the existence of God and the possibility that life and thought themselves might be explained by purely natural and material agencies. D’Alembert proclaims himself unable to decide the issue; he is, in his own words, “a sceptic.” When Diderot argues that scepticism about categorical naturalism and God would lead him to absurd conclusions, his d’Alembert replies: “You are wrong; sceptic I shall go to bed and sceptic I shall arise.” Diderot answers that such scepticism is impossible. For Diderot, in the Rêve de d’Alembert, scepticism was simply a lack of mental concentration: forgetting the reasons that rightly convinced him, d’Alembert, contemplating an objection, simply gave up focusing on the stronger answer. There was not a single controverted question on which a man truly could be “with an equal and rigorous measure of reason for and against” and, “this being the case, there is no real sceptic…. The scale is thus never equally balanced.” When d’Alembert insists that this balance tips one way in the morning and another in the afternoon, Diderot answers, “That is to say, you are dogmatically for, in the morning, and you are dogmatically against, in the afternoon,” and he concludes that “our true opinion is not that in which we never have vacillated, but that to which we most usually return.” The alternative, he noted, was “Buridan’s donkey.” (The hypothetical donkey attributed to Jean Buridan was dying equally from hunger and thirst. Placed equidistant between hay and water, it was unable to choose one over the other and died from both dehydration and starvation. For the record, Jean Buridan himself never offered such an example; it was raised by critics who opposed his denial of free will.) D’Alembert conveniently agrees that Diderot is correct about the impossibility of authentic scepticism.Footnote 2

Diderot is a conceptually playful thinker, however, and his true sense of the status of the naturalistic propositions of the Rêve de d’Alembert—the farthest he will go—is expressed by Théophile de Bordeu, his primary voice in what follows. Asked by Julie if these materialist explanations of species, thought, and individuation make sense, Bordeu replies that such philosophy is an effort at system-building in the present day, but will most probably be confirmed by future human knowledge: “That is elevated philosophy; overly systematic at this time, I believe that the more human knowledge will progress, the more this philosophy will be confirmed.”Footnote 3

If such views were not yet confirmed, however, why not wait for that progress in human knowledge? Why bother, Julie asks at a critical moment of the dialogue, with what seem unanswerable questions? So many of these arguments are “so obscure that one can’t see a thing in them,” and “they are all perfectly useless.” In reply, Bordeu perhaps gives the game away. The stakes, he notes, are so very, very high: “Do you believe, Mademoiselle, that it is a matter of indifference to deny or to accept a Supreme Intelligence?” Obviously that was the crucial question, but could one possibly decide about God without knowing how to look at questions such as “the eternity of matter and its properties, the distinction between the two substances, the nature of man, and the production of animals.” Julie concedes that, but asks what importance these latter questions could have “if there is no way for me to clarify them?” For Bordeu—that is, for Diderot—only the positing of a materialist, categorically naturalistic science opened the road to a future of such vital knowledge: “And how will you know that if you don’t examine them?”Footnote 4

In Diderot’s mind, there were no demonstrative proofs of materialism, but, rather, a sense that it was the only possible route to understanding what could be understood, however limited, about the conditions in which we found ourselves and the only means to advance in that understanding. As he put it in the atheistic Additions aux Pensées philosophiques, supplementing his earlier deistic Pensées philosophiques, “Lost in an immense forest in the middle of the night, I have only a small lantern to guide me. Along comes a stranger who says to me, ‘My friend, blow out the candle in order better to find your way.’ That stranger is a theologian.” Diderot understood well that reason and experience gave us only “a small lantern.”Footnote 5 As he wrote with full force in his De l’Interprétation de la nature:

The understanding has its prejudices; the senses, their uncertainty; the memory, its limits; the imagination, its glow; instruments, their imperfection. Phenomena are infinite; causes, hidden; forms, perhaps ephemeral. Against so many obstacles that we find in ourselves, and that nature opposes to us outside ourselves, we have only a slow, gradual experience and only a limited power of reflection. There are the levers with which philosophy proposes to move the world.Footnote 6

That is not a thinker without a sceptical awareness, to say the least. As the narrator notes in Jacques le fataliste—here, one thinks, Diderot himself (though his narrator is indeed a character in the tale)—concerning the entire issue of determinism and the world, “You can form an idea, reader, to what point I could push this conversation on a subject on which one has talked so much, and written so much, over 2,000 years, without ever having advanced it a single step.”Footnote 7

Consider Jacques le fataliste et son maître, in which all of the problems of determinism are laid out with such dramatic human and moral force, and in which, yet more strikingly, the narrator argues that human beings do not actually live according to what they claim to believe, whether Christian or determinist. Jacques believes precisely what Bordeu argues in the Rêve de d’Alembert, that any act made by a person is necessary, because that person is the single combination of causes in a circumstance that is determined. The materialist Rêve is explicit about this: “One is happily or unhappily born; one is imperceptibly carried along by the general current that leads one person to glory, another to ignominy.”Footnote 8 Jacques is convinced of that: “He [Jacques] believed that a man is moved as necessarily to glory or to ignominy as a ball with consciousness of itself follows the slope of a mountain.” It is true of any man, given naturalistic determinism, that “He did only what it was necessary for him to do.” The problem, however, is that Jacques cannot live by such a belief. Given his philosophy, the narrator assures us, Jacques should not have rejoiced or suffered, in the supposed example of Spinoza, but, in fact, “he behaved about like you or I.” Indeed, he tried as best he could to prevent evil and to aid those in distress, even though such actions made absolutely no sense to him.Footnote 9 In Le neveu de Rameau, it is the sceptical and cynical nephew, “lui,” who cannot be satisfactorily answered by Diderot, the “moi.” The philosopher is the freak of nature who cannot understand the world because he filters it through the prism of his own aberrant self and mind.

Holbach, to say the least, did not share Diderot’s conceptual playfulness. Diderot always was in dialogue with himself, delighting in paradox, and delighting in every objection that he could formulate well against his own philosophy. As Diderot understood, philosophy is often temperament, and his inner life was that dialectic, always arguing with his own mind, though the image of “this little lantern” of reason and experience, in the dark, immense forest, was clearly the dependence he recognized the most. The Encyclopédie of all human knowledge and intellectual accomplishment was not a philosophical sceptic’s undertaking, to say the least, however much Diderot understood the lure of scepticism.

Holbach, however, indeed appears dogmatic, in the philosophical sense, without reserve. “Necessity,” he wrote in his Essais sur les prejugés, “leads men sooner or later to truth.”Footnote 10 The notion of paradox was absurd: it was either “a truth opposed to the prejudices of the vulgar,” or, if the fruit of serious reasoning, “what is today a paradox for us will be for posterity a demonstrated truth.”Footnote 11 As he wrote throughout his most celebrated work, the Système de la nature, man was unhappy only because he knew nature badly, but he could have rightful knowledge of the natural world.Footnote 12 There was no distinction between the moral and the physical world, and “it is thus to physics and to experience that man must have recourse in all of his inquiries. It is they that he must consult in his religion, in his morality, in his legislation, in his political government, in the sciences and in the arts, in his pleasures, and in his pains.”Footnote 13 It is certain that matter and motion account for all phenomena and that gravitation explains all motion.Footnote 14 All of our faculties of intelligence knowably are derived from sensation and body alone.Footnote 15 Indeed, “it is purely by mechanistic means that we can explain phenomena, both physical and moral.”Footnote 16 The world is knowably and categorically determined by a sequence of physical causes.Footnote 17 Not only is “the system of fatalism” not dangerous, but it is essential to human well-being, and “ideas that are true or founded on nature are the only remedies to the ills of men.”Footnote 18 In short, the materialist atheist is a “physicist [physicien]” who believes that “without resorting to a chimerical cause, one can explain everything by the laws of motion alone, by the relationships subsisting among beings, by their affinities, their analogies, their actions and their repulsions, their proportions, their compositions, and their decompositions.”Footnote 19 It seems difficult to find any scepticism in that.

For Holbach, however, the human mind was profoundly weak and drawn to error, and whatever the truth about the world, the mind’s ability to grasp that truth was deeply uncertain. Men were, also as described in the Système de la nature, creatures that prejudices “render obstinately determined to harm themselves,” accustomed “to fear reason, to look at truth as dangerous.”Footnote 20 Men wanted to believe what was false, even when it harmed them, and especially when it was reinforced by education and by their fears: “The clearest truths are forced to fail when countered by enthusiasm, habituation, and fear.” Most human beings were “infatuated by the marvelous, disdaining what is simple and easy to understand.” They wanted gods who would protect them, and “neither experience nor reflection can disabuse them of these.” We were complicit in our deception.Footnote 21

For Holbach, the idea of God “does not appear to be of a nature that can be uprooted” from the minds of more than a few, and atheism, alas “is thus not made … for the vulgar.”Footnote 22 Tragically, men’s eyes are more comfortable in shadows than in light, and they hate the philosophical atheist for trying to remove their blindfolds.Footnote 23 As he wrote in Le Bon-Sens, anyone who combats superstition and belief in God “with the arms of reason resembles a man who uses a sword to kill fruit flies.” The philosopher’s blow might be impressive, but after it, the fruit flies of superstition “take once again, in the heads of folks, the place from which one believed to have banished them.”Footnote 24 Elsewhere, I have argued that this last view was sceptical only in the sense that scepticism implied a sense of the weakness of the human mind.Footnote 25 This understates the case. If there is something inherent and ineradicable in the human mind that, in general, prevents it from knowing the truth, the sceptical argument is rather strong.

The philosopher who knew Diderot and Holbach intimately, Jacques-André Naigeon, has not fared well in either French studies or the history of philosophy. Scholars seem upset that Diderot chose him as his literary executor—how could Diderot have misjudged so badly? they ask in effect—leaving it to Naigeon to publish posthumously his most materialist and adventuresome works at Naigeon’s discretion. Naigeon was also Holbach’s closest collaborator, working on first the “atheized” translations and editions of British deists critical of Christianity, and then collaborating on some of the most important atheistic work of all. The French publishing world chose Naigeon as editor of some of the most significant collected works and critical editions of the late eighteenth and early-nineteenth centuries. He also was chosen as editor of the three-volume Philosophie ancienne et moderne at the dawn of the Revolution, a remarkable work that replicated some of the articles on philosophy in Diderot’s original Encyclopédie, with critical emendations and supplements, and that offered a large number of wholly original essays on ancient and modern philosophers, and on philosophical themes, all written from an atheistic, naturalistic, materialist perspective (no small danger once Robespierre pronounced atheism “aristocratic” and proclaimed the cult of the Supreme Being). Naigeon is best known as the author of the occasionally infuriating but substantively important and profoundly illuminating Mémoires historiques et philosophiques sur la vie et les ouvrages de Diderot.Footnote 26

Commenting, in the latter work, on Diderot’s statement, through Bordeu, that the theories of the Rêve de d’Alembert were system-building now, but that one day they would be confirmed by advances in human knowledge, Naigeon observed that such explanations of phenomena “should never be proposed with this confidence that facts susceptible of demonstration inspire.” Diderot understood that precisely, Naigeon added, and it was not accidental, he argued, that the speculations occur in the mouth of a dreaming man during an agitated night.Footnote 27

In his atheistic history of philosophy, Naigeon’s views of scepticism shed light, perhaps quite significantly, if unintentionally so, on the uneasy relationship between late-Enlightenment atheism and scepticism. In his own article on the ancient “Académiciens”—indeed, in the opening section of his first piece—Naigeon proclaimed Socrates to be the true father of philosophy. He saw deeply, Naigeon judged, “but Socrates himself believed perhaps, like the [sceptical] academicians who followed him, that the best manner of teaching was to advance the for and the against, and he made great use of this method, never affirming anything with assurance, and declaring that he only searched, and that all of his science was reduced to knowing that he knew nothing.” Such a method, for Naigeon, led to a great diversity of views, which, he argued, was precisely the value of philosophy.Footnote 28

One is tempted, Naigeon observed, to view Plato as one of the dogmatic as opposed to sceptical philosophers, because he had “fixed and finalized opinions.” However, one could portray him just as easily as “one of the sceptics, who doubted everything, made inquiries into everything, examined everything, while affirming nothing, and being convinced of nothing.” The issue, Naigeon argued, and perhaps this is precisely how Diderot and Holbach saw themselves, is not one of holding certain beliefs as dogmas or not, “for nothing prevents a sceptic from having dogmas, provided that he considers them as probable, not as certain.”Footnote 29

Sextus Empiricus, for Naigeon, had understood scepticism well: it was “a manner or force that opposes in all ways, against each other, things that are sensed and things that are understood,” since, for Naigeon, the great philosophical dilemma was somehow to try to relate coherently sensations and intelligible ideas. He admired Diogenes Laertius’ account of Pyrrhonism, namely, that it was an awareness of the confusion of all comparisons between what struck our senses, on the one hand, and the ideas of our minds, on the other. The authentic sceptic, Naigeon asserted, did not deny appearances, or a world of appearances in which we had to live, but denied the coherence of what we asserted about those appearances.Footnote 30

In a signal article on the philosophy of David Hume, Naigeon sought to render forcefully and approvingly Hume’s “sceptical doubts concerning the operations of the understanding.” What we knew about all sensory phenomena was categorically uncertain, which we saw when we examined, by contrast, the relationship of ideas—geometry or pure logic—where certainty was attainable even about things that did not exist. In matters of fact concerning the world in which we find ourselves, we deal with cause and effect, but “there is no object that manifests, by its sensible qualities, the causes that produced it nor the effects that it will produce in its turn.” Experience never allows certainty, because at best we can infer “a small number of general causes, but the causes of these causes will escape us always, and we never find a satisfying explanation of them.” Philosophy, Naigeon wrote, is, in the final analysis, humbling: “the whole result of philosophy is to teach us the degree to which we know few things, and to convince us of our insufficiency. In vain we revolt against this, make efforts to surmount these difficulties, or to avoid them. Whatever detour we take, they halt our passage.”Footnote 31

We try to use probabilities of power, force, energy, and necessary connection, but “metaphysics has nothing more obscure or more uncertain than the ideas of power, of force, of energy, of necessary connection, ideas, nevertheless, that we need at every moment of our inquiries.” We want to understand the world, but “the scene of the universe is subjected to a perpetual change, objects following each other in a continual succession; but the power, or the force, that animates the whole machine is hidden from our gaze; and the sensible qualities of bodies have nothing that can disclose them to us.” Speculate as we will, we know neither liberty nor necessity, because we do not understand the force of the world. We must deal with the world of appearances, and we cannot go beyond it: “Experience becomes silent here, and must become silent. Nothing can be present to the mind beyond perception; and given that, it is impossible that we could have an experience of their connection with objects. It is thus without any reasonable foundation that this connection can be assumed.”Footnote 32

Here, Naigeon, in a footnote of his own, gave voice to Hume’s well-known footnote on Berkeley, in the Essay Concerning Human Understanding, arguing that it was a great irony that the latter believed that he wrote against scepticism, when his irrefutable arguments lead, in fact, only and precisely to scepticism. Berkeley’s demonstrations of the unknowability of a cause that corresponded to and caused our sensations, in Naigeon’s translation, “are the best lessons of scepticism that one can encounter, either in the ancient philosophers, or in the moderns, without even making an exception for [Pierre] Bayle.”Footnote 33

Nonetheless, Naigeon concluded, echoing Jacques in Diderot’s Jacques le fataliste, life trumps philosophy, and in the actual life of a Pyrrhonian, the latter acts the same as “those who do not bother themselves with philosophical inquiry.” Even though nothing about matters of fact and human existence is “susceptible of demonstration,” we live our lives based on our experience of the world.Footnote 34

For both Holbach and Naigeon, atheism was essential to liberate men and women from what they saw as a terrible suffering caused by religion, but it could not offer either a satisfying or even a comforting view of the universe. For both, awareness of the world in which we found ourselves was a sobering knowledge of our fragility, exposure, intellectual weakness, and lack of absolutes. It was a constant of Naigeon’s philosophy that a purely speculative atheistic materialism could be resisted by ingenious hypotheses, and that only an empirical, “experimental” atheism had compelling, philosophical force. He withheld certainty, however, from empirical knowledge, and explicitly agreed with Berkeley that fallibility began with any judgment beyond the mere recording of the immediate objects of perception in ideas. He accepted, indeed insisted, that we reasoned only from appearances, never from knowledge of real qualities. He declared himself disappointed by Hume’s willingness to remain in a state of sceptical suspension of judgment about theism, but, rare for Naigeon, he did not propose a single philosophical argument against Hume’s Dialogues, except to complain that they contained nothing concrete and practical that could be drawn from them.Footnote 35

In the Philosophie ancienne et moderne, part of the celebrated Encyclopédie méthodique, Naigeon, in an otherwise expansively laudatory article, criticized Bacon’s use of the argument from design, arguing against him that the spectacle of nature proved nothing because, to speak accurately, there was nothing inherently “beautiful” or “horrible” in the universe. For men who “coexist happily” with nature, the world indeed will appear an example of art; for men who “coexist painfully” with nature, the very same sequence of causes and effects will appear imperfect and unplanned. It would be consoling, Naigeon wrote, to imagine a universal and eternal coexistence, but nothing in our experience makes that inductively plausible or even, in the final analysis, possible.Footnote 36

Indeed, he wrote to Diderot in 1766, it was merely a “sweet error” and a “beautiful chimera” to believe that humanity could be improved in any fundamental way. Footnote 37 In Naigeon’s system, only matter is permanent, and all of its particular forms, of which man (or any species) is only one, perish after a cosmically brief coexistence. Atheism, he believed, with Holbach, could free us from the immediate tyranny of the priests, the sad superstitions of religion, and the denials of the senses imposed by the churches, but atheism could not resolve the ultimate mysteries, pains, and insecurities of the human condition. Indeed, it was precisely not to face those mysteries, pains, and insecurities that men tended to religion in the first place. The appeal of such religion appeared permanent.

In his commentary on Diderot’s Rêve de d’Alembert, after the highest praise of Diderot’s materialism, Naigeon wrote with a rare frankness about the rarity of minds capable of agreeing with materialist views. It is a passage that merits quotation in full:

Indeed, we should not conceal it: the philosophy taught in the two Dialogues is suitable only for a very small number of privileged beings. It requires too much repeated study, meditation, and acquisition of knowledge for the principles that serve as its foundation ever to be granted in all of their consequences, not only, I say, by the vulgar, but even by those who, placed in more fortunate circumstances and with more means of instructing themselves, have, in general, in these matters, neither a faith less blind than that of the people nor an incredulity more purposeful and reasoned. This consideration should reassure the founders of the most false religions about the duration of their empire. It can end, unfortunately, only with the human species, because it is founded upon the natural laziness and inertia of man, upon this love for the marvelous, upon an ignorance that renders him fearful and superstitious, and, above all, upon this need, so pressing, so imperious, and virtually universal, to believe.Footnote 38

These dispositions, Naigeon wrote, can momentarily and ephemerally weaken in certain circumstances,

But being inherent in human nature, and never varying in each individual except by their degree of energy, they will necessarily subject, in all times and in all nations, weak mortals to the yoke that the fanatics will wish to impose on them, however absurd the superstitions that will replace, in the succession of centuries, those that already have reigned on earth and that have covered it with shadows and with crimes.Footnote 39

If that is not philosophical scepticism, we need perhaps to expand the meaning of that term. At the very least, the relationship between late Enlightenment atheism and scepticism ought to be as much a part of Enlightenment history as are the links traditionally drawn between such atheism and the materialistic positivism and scientism of the nineteenth century.