A few more pages must be given to one or two of Diderot’s writings which have not hitherto been mentioned. An exhaustive survey of his works is out of the question, nor would any one be repaid for the labour of criticism. A mere list of the topics that he handled would fill a long chapter. A redaction of a long treatise on harmony, a vast sheaf of notes on the elements of physiology, a collection of miscellanea on the drama, a still more copious collection of miscellanea on a hundred points in literature and art, a fragment on the exercise of young Russians, an elaborate plan of studies for a proposed Russian University,—no less panurgic and less encyclopÆdic a critic than Diderot himself could undertake to sweep with ever so light a wing over this vast area. Everybody can find something to say about the collection of tales, in which Diderot thought that he was satirising the manners of his time, after the fashion of Rabelais, Montaigne, La Mothe-le-Vayer, and Swift. But not everybody is competent to deal, for instance, with the five memoirs on different subjects in mathematics (1748), with which Diderot hoped to efface the scandal of his previous performance.

I.

Decidedly the most important of the pieces of which we have not yet spoken must be counted the Thoughts on the Interpretation of Nature (1754). His study of Bacon and the composition of the introductory prospectus of the EncyclopÆdia had naturally filled Diderot’s mind with ideas about the universe as a whole. The great problem of man’s knowledge of this universe,—the limits, the instruments, the meaning of such knowledge, came before him with a force that he could not evade. Maupertuis had in 1751, under the assumed name of Baumann, an imaginary doctor of Erlangen, published a dissertation on the Universal System of Nature, in which he seems to have maintained that the mechanism of the universe is one and the same throughout, modifying itself, or being modified by some vital element within, in an infinity of diverse ways.[207] Leibnitz’s famous idea, of making nature invariably work with the minimum of action, was seized by Maupertuis, expressed as the Law of Thrift, and made the starting-point of speculations that led directly to Holbach and the System of Nature.[208] The Loi d’Epargne evidently tended to make unity of all the forces of the universe the keynote or the goal of philosophical inquiry. At this time of his life, Diderot resisted Maupertuis’s theory of the unity of vital force in the universe, or perhaps we should rather say that he saw how open it was to criticism. His resistance has none of his usual air of vehement conviction. However that may be, the theory excited his interest, and fitted in with the train of meditation which his thoughts about the EncyclopÆdia had already set in motion, and of which the PensÉes Philosophiques of 1746 were the cruder prelude.

The Thoughts on the Interpretation of Nature are, in form as in title, imitated from those famous Aphorismi de Interpretatione NaturÆ et Regni Hominis, which are more shortly known to all men as Bacon’s Novum Organum.[209] The connection between the aphorisms is very loosely held. Diderot began by premising that he would let his thoughts follow one another under his pen, in the order in which the subjects came up in his mind; and he kept his word. Their general scope, so far as it is capable of condensed expression, may be described as a reconciliation between the two great classes into which Diderot found thinkers upon Nature to be divided; those who have many instruments and few ideas, and those who have few instruments and many ideas,—in other words, between men of science without philosophy, and philosophers without knowledge of experimental science.

In the region of science itself, again, Diderot foresees as great a change as in the relations between science and philosophy. “We touch the moment of a great revolution in the sciences. From the strong inclination of men’s minds towards morals, literature, the history of nature and experimental physics, I would almost venture to assert that before the next hundred years are over, there will not be three great geometers to be counted in Europe. This science will stop short where the Bernouillis, the Eulers, the Maupertuis, the Clairauts, the Fontaines, the D’Alemberts, the Lagranges have left it. They will have fixed the Pillars of Hercules. People will go no further.” Those who have read Comte’s angry denunciations of the perversions of geometry by means of algebra, and of the waste of intellectual force in modern analysis,[210] will at least understand how such a view as Diderot’s was possible. And no one will be likely to deny that, whether or not the pillars of the geometrical Hercules were finally set a hundred years ago, the great discoveries of the hundred years since Diderot have been, as he predicted, in the higher sciences. The great misfortune of France was that the supremacy of geometry coincided with the opening of the great era of political discussion. The definitions of Montesquieu’s famous book, which opened the political movement in literature, have been shown to be less those of a jurisconsult than of a geometer.[211] Social truths, with all their profound complexity, were handled like propositions in Euclid, and logical deductions from arbitrary premises were treated as accurate representations of real circumstance. The repulse of geometry to its proper rank came too late.

Comte always liberally recognised Diderot’s genius, and any reader of Comte’s views on the necessities of subjective synthesis will discern the germ of that doctrine in the following remarkable section:

We cannot wonder that when Comte drew up his list of the hundred and fifty volumes that should form the good Positivist’s library in the nineteenth century, he should have placed Diderot’s Interpretation of Nature on one side of Descartes’ Discourse on Method, with Bacon’s Novum Organum on the other.

The same spirit finds even stronger and more distinct expression in a later aphorism:—“Since the reason cannot understand everything, imagination foresee everything, sense observe everything, nor memory retain everything; since great men are born at such remote intervals, and the progress of science is so interrupted by revolution, that whole ages of study are passed in recovering the knowledge of the centuries that are gone,—to observe everything in nature without distinction is to fail in duty to the human race. Men who are beyond the common run in their talents ought to respect themselves and posterity in the employment of their time. What would posterity think of us if we had nothing to transmit to it save a complete insectology, an immense history of microscopic animals? No—to the great geniuses great objects, little objects to the little geniuses” (§ 54).

Diderot, while thus warning inquirers against danger on one side, was alive to the advantages of stubborn and unlimited experiment on the other. “When you have formed in your mind,” he says, “one of those systems which require to be verified by experience, you ought neither to cling to it obstinately nor abandon it lightly. People sometimes think their conjectures false, when they have not taken the proper measures to find them true. Obstinacy, even, has fewer drawbacks than the opposite excess. By multiplying experiments, if you do not find what you want, it may happen that you will come on something better. Never is time employed in interrogating nature entirely lost” (§ 42). The reader will not fail to observe that this maxim is limited by the condition of verifiableness. Of any system that could not be verified by experience Diderot would have disdained to speak in connection with the interpretation of nature.

This, of course, did not prevent him from hypothesis and prophecy which he himself had not the means of justifying. For example, he said that just as in mathematics, by examining all properties of a curve we find that they are one and the same property presented under different faces, so in nature when experimental physics are more advanced, people will recognise that all the phenomena, whether of weight, or elasticity, or magnetism, or electricity, are only different sides of the same affection (§ 44). But he was content to leave it to posterity, and to build no fabric on unproved propositions.

In the same scientific spirit he penetrated the hollowness of every system dealing with Final Causes:

The world has advanced rapidly along this path since Diderot’s day, and has opened out many new and unsuspected meanings by the way. Perhaps the advance has been less satisfactory in working out, in a scientific way, the philosophy that is implied in the following adaptation of the Leibnitzian and Maupertuisian suggestion of the law of economy in natural forces:—“Astonishment often comes from our supposing several marvels, where in truth there is only one; from our imagining in nature as many particular acts as we can count phenomena, whilst nature has perhaps in reality never produced more than one single act. It seem even that, if nature had been under the necessity of producing several acts, the different results of such acts would be isolated; that there would be collections of phenomena independent of one another, and that the general chain of which philosophy assumes the continuity, would break in many places. The absolute independence of a single fact is incompatible with the idea of an All; and without the idea of a Whole, there can be no Philosophy” (§ 11).

At length Diderot concludes by a series of questions which he thinks that philosophers may perhaps count worthy of discussion. What is the difference, for example, between living matter and dead? Does the energy of a living molecule vary by itself, or according to the quantity, the quality, the forms of the dead or living matter with which it is united? We need not continue the enumeration, because Diderot himself suddenly brings them to an end with a truly admirable expression of his sense of how unworthy they are of the attention of serious men, who are able to measure the difference between a wise and beneficent use of intelligence, and a foolish and wasteful misuse of it. “When I turn my eyes,” he says, “to the works of men, and see the cities that are built on every side, all the elements yoked to our service, languages fixed, nations civilised, harbours constructed, lands and skies measured—then the world seems to me very old. When I find man uncertain as to the first principles of medicine and agriculture, as to the properties of the commonest substances, as to knowledge of the maladies that afflict him, as to the pruning of trees, as to the best form for the plough, then it seems as if the earth had only been inhabited yesterday. And if men were wise, they would at last give themselves up to such inquiries as bear on their wellbeing, and would not take the trouble to answer my futile questions for a thousand years at the very soonest; or perhaps, even, considering the very scanty extent that they occupy in space and time, they would never deign to answer them at all.”

II.

In 1769 Diderot composed three dialogues, of which he said that, with a certain mathematical memoir, they were the only writings of his own with which he was contented. The first is a dialogue between himself and D’Alembert; the second is D’Alembert’s Dream, in which D’Alembert in his sleep continues the discussion, while Mdlle. Lespinasse, who is watching by his bedside, takes down the dreamer’s words; in the third, Mdlle. Lespinasse and the famous physician, Bordeu, conclude the matter.[213] It is impossible, Diderot said to Mdlle. Voland, to be more profound and more mad: it is at once a supreme extravagance, and the most deep-reaching philosophy. He congratulated himself on the cleverness of placing his ideas in the mouth of a man who dreams, on the ground that we must often give to wisdom the air of madness, in order to secure admittance. Mdlle. Lespinasse was not so complacent. She made D’Alembert insist that the dialogue should be destroyed, and Diderot believed that he had burned the only existing copy. As a matter of fact, the manuscript was not published until 1830, when all the people concerned had long been reduced to dust. There are five or six pages, Diderot said to Mdlle. Voland, which would make your sister’s hair stand on end. A man may be much less squeamish than Mdlle. Voland’s sister, and still pronounce the imaginative invention of D’Alembert’s Dream, and the sequel, to be as odious as anything since the freaks of filthy Diogenes in his tub. Two remarks may be made on this strange production. First, Diderot never intended the dialogues for the public eye. He would have been as shocked as the Archbishop of Paris himself, if he had supposed that they would become accessible to everybody who knows how to read. Second, though they are in form the most ugly and disgusting piece in the literature of philosophy, they testify in their own way to Diderot’s sincerity of interest in his subject. Science is essentially unsparing and unblushing, and D’Alembert’s Dream plunged exactly into those parts of physiology which are least fit to be handled in literature. The attempt to give an air of polite comedy to functions and secretions must be pronounced detestable, in spite of the dialectical acuteness and force with which Diderot pressed his point.

It would be impossible, in a book not exclusively designed for a public of professors, to give a full account of these three dialogues. It is indispensable to describe their drift, because it is here that Diderot figures definitely as a materialist. Diderot was in no sense the originator of the French materialism of the eighteenth century. He was preceded by Maupertuis, by Robinet, and by La Mettrie; and we have already seen that when he composed the Thoughts on the Interpretation of Nature (1754), he did not fully accept Maupertuis’s materialistic thesis. Lange has shown that at a very early period in the movement the most consistent materialism was ready and developed, while such leaders of the movement as Voltaire and Diderot still leaned either on deism, or on a mixture of deism and scepticism.[214] The philosophy of D’Alembert’s Dream is definite enough, and far enough removed alike from deism and scepticism.

“The thinking man is like a musical instrument. Suppose a clavecin to have sensibility and memory, and then say whether it would not repeat of itself the airs that you have played on its keys. We are instruments endowed with sensibility and memory. Our senses are so many keys, pressed by the nature that surrounds them, and they often press one another; and this, according to my judgment, is all that passes in a clavecin organised as you and I are organised.

“There is only one substance in the world. The marble of the statue makes the flesh of the man, and conversely. Reduce a block of marble to impalpable powder; mix this powder with humus, or vegetable earth; knead them well together; water the mixture; let it rot for a year, two years—time does not count. In this you sow the plant, the plant nourishes the man, and hence the passage from marble to tissue.

“Do you see this egg? With that you overturn all the schools of theology and all the temples of the earth. It is an insensible mass before the germ is introduced into it; and, after the germ is introduced, there is still an insensible mass, for the germ itself is only an inert fluid. How does this mass pass to another organisation, to life, to sensibility? By heat. What will produce heat? Movement. What will be the successive effects of movement? First, an oscillating point, a thread that extends, the flesh, the beak, and so forth.”

Then follows the application of the same ideas to the reproduction of man—a region whither it is not convenient to follow the physiological inquirer. The result as to the formation of the organic substance in man is as unflinching as the materialism of BÜchner.

The third dialogue we must leave. The fact that German books are written for a public of specialists allows Dr. Rosenkranz to criticise these dialogues with a freedom equal to Diderot’s own, and his criticism is as full as usual of candour, patience, and weight. An English writer must be content to pass on, and his contentment may well be considerable, for the subject is perhaps that on which, above all others, it is most difficult to say any wise word.

III.

The Plan of a University for the Government of Russia was the work of Diderot’s last years, but no copy of it was given to the public before 1813-14, when M. Guizot published extracts from an autograph manuscript confided to him by Suard. Diderot, with a characteristic respect for competence, with which no egotism can ever interfere in minds of such strength and veracity as his, began by urging the Empress to consult Ernesti of Leipsic, the famous editor of Cicero, and no less famous in his day (1707-1781) for the changes that he introduced into the system of teaching in the German universities. Of Oxford and Cambridge Diderot spoke more kindly than they then deserved.

The one strongly marked idea of the plan is what might have been expected from the editor of the EncyclopÆdia, namely, the elevation of what the Germans call real or technological instruction, and the banishment of pure literature as a subject of study from the first to the last place in the course. In the faculty of arts the earliest course begins with arithmetic, algebra, the calculation of probabilities, and geometry. Next follow physics and mechanics. Then astronomy. Fourthly, natural history and experimental physics. In the fifth class, chemistry and anatomy. In the sixth, logic and grammar. In the seventh, the language of the country. And it was not until the eighth, that Greek and Latin, eloquence and poetry, took their place among the objects or instruments of education. Parallel with this course, the student was to follow the first principles of metaphysics, of universal morality, and of natural and revealed religion. Here, too, history and geography had a place. In a third parallel, perspective and drawing accompanied the science of the first, and the philosophy and history of the second.

In the thorny field of religious instruction, Diderot expresses no opinion of his own, beyond saying that it is natural for the Empress’s subjects to conform to her way of thinking. As her majesty thinks that the fear of pains to come has much influence on men’s actions, and is persuaded that the total of small daily advantages produced by belief outweighs the total of evils wrought by sectarianism and intolerance, therefore students ought to be instructed in the mystery of the distinction of the two substances, in the immortality of the soul, and so forth.[216]

There is a story that one evening at St. Petersburg, Diderot was declaiming with stormy eloquence against the baseness of those who flatter kings; for such, he said, there ought to be a deeper and a fiercer hell. “Tell me, Diderot,” said the Empress by and by, “what they say in Paris about the death of my husband.” Instead of telling her the plain truth that everybody said that Peter had been murdered by her orders, the philosopher poured out a stream of the smoothest things. “Come now,” said Catherine suddenly, “confess, if you are not walking along the path that leads to your deep hell, you are certainly coming very close to purgatory.” Diderot’s elaborate concessions to her majesty’s political religion would, it is to be feared, have brought him still further in the same sulphureous track.

As we have often had to bewail Diderot’s diffuseness, it is as well to remark that a long passage in the sketch of which we are speaking shows how close and concentrated he could be upon occasion. The two pages in which he demolishes the incorrigible superstition about Latin and Greek,[217] contain a thoroughly exhaustive summary of all the arguments and the answers. In the immense discussion about Latin and Greek that has taken place in the hundred years since Diderot’s time, it is tolerably safe to say that not a single point has been brought forward which Diderot did not in these most pithy and conclusive pages attempt to deal with. He winds up with the position that, even for the man of letters, the present system of teaching Latin and Greek is essentially sterile. I am perfectly sure, he says, that Voltaire, who is not exactly a mediocrity as a man of letters, knows extremely little Greek, and that he is not twentieth nor even hundredth among the Latinists of the day.[218]

Following this sketch is printed a letter to the Countess of Forbach on the education of children. It is full of rich wisdom on its special subject. Nobody can read it without feeling that quality in Diderot which made his friends love him. And we see how, when he was called to practical counsel, he banished into their own sphere the explosive paradoxes with which he delighted to amuse his hours of speculative dreaming.

IV.

Romilly has told us that Diderot was bent on converting him from the error of his religious ways, and with that intention read to him a Conversation with the MarÉchale de——.[219] It is believed to be an idealised version of a real conversation with Madame de Broglie, and was first printed, almost as soon as written (1777), in the correspondence in which MÉtra, in imitation of Grimm, informed a circle of foreign subscribers what was going on in Paris. The admirers of Diderot profess to look on this Conversation as one of the most precious pearls in his philosophic casket. It turns upon the conditions of belief and unbelief, represented by the two interlocutors respectively, and is a terse and graphic summary of the rationalistic objections to the creed of the church. The most conspicuous literary passage in it is a parable which has been attributed to Rousseau, but with which Rousseau had really nothing to do, beyond reproducing the spirit of its argument in the ever famous creed of the Savoyard Vicar.

A young Mexican, tired of his work, was sauntering one day on the seashore. He spied a plank, with one end resting on the land, and the other dipping into the water. He sat down on the plank, and there gazing over the vast space that lay spread out before him, he said to himself: “It is certain that my old grandmother is talking nonsense, with her history of I know not what inhabitants, who, at I know not what time, landed here from I know not where, from some country far beyond our seas. It is against common sense: do I not see the ocean touch the line of the sky? And can I believe, against the evidence of my senses, an old fable of which nobody knows the date, which everybody arranges according to his fancy, and which is only a tissue of absurdities, about which people are ready to tear out one another’s eyes.” As he was reasoning in this way, the waters rocked him gently on his plank, and he fell asleep. As he slept, the wind rose, the waves carried away the plank on which he was stretched out, and behold our youthful reasoner embarked on a voyage.

La MarÉchale.—Alas, that is the image of all of us; we are each on our plank; the wind blows, and the flood carries us away.C.—He was already far from the mainland when he awoke. No one was ever so surprised as our young Mexican, to find himself out on the open sea, and he was mightily surprised, too, when having lost from sight the shore on which he had been idly walking only an instant before, he saw the sea touching the line of the sky on every side. Then he began to suspect that he might have been mistaken, and that, if the wind remained in the same quarter, perhaps he would be borne to that very shore and among those dwellers on it, about whom his grandmother had so often told him.

La MarÉchale.—And of his anxiety you say nothing.

C.—He had none. He said to himself: “What does it matter, provided that I find land? I have reasoned like a giddy-pate, granted; but I have been sincere with myself, and that is all that can be required of me. If it is no virtue to have understanding, at any rate it is no crime to be without it.” Meanwhile the wind continued, the man and the plank floated on, and the unknown shore came into sight. He touched it, and behold him again on land.

La MarÉchale.—Ah, we shall all of us see one another there, one of these days.

C.—I hope so, madam; wherever it may be, I shall always be very proud to pay you my homage. Hardly had he quitted his plank, and put his foot on the sand, when he perceived a venerable old man standing by his side. He asked him where he was, and to whom he had the honour of speaking. “I am the sovereign of the country,” replied the old man; “you have denied my existence?”—“Yes, it is true.”—“And that of my empire?”—“It is true!”—“I forgive you, because I am he who sees the bottom of all hearts, and I have read at the bottom of yours that you are of good faith; but the rest of your thoughts and your actions are not equally innocent.” Then the old man, who held him by the ear, recalled to him all the errors of his life; and as each was mentioned, the young Mexican bowed himself upon the ground, beat his breast, and besought forgiveness.