'Let us return to the soul of Julius Caesar, and call it an immaterial automaton (M. Leibniz's own phrase), and compare it with an atom of Epicurus; I mean an atom surrounded with a vacuum on all sides, and which will never meet any other atom. This is a very just comparison: for this atom, on the one hand, has a natural power of moving itself and exerts it without any assistance, and without being retarded or hindered by anything: and, on the other hand, the soul of Caesar is a spirit which has received the faculty of producing thoughts, and exerts it without the influence of any other spirit or of any body. It is neither assisted nor thwarted by anything whatsoever. If you consult the common notions and the ideas of order, you will find that this atom can never stop, and that having been in motion in the foregoing moment, it will continue in it at the present moment and in all the moments that shall follow, and that it will always move in the same manner. This is the consequence of an axiom approved by M. Leibniz: since a thing does always remain in the same state wherein it happens to be, unless it receives some alteration from some other thing … we conclude, says he, not only that a body which is at rest will always be at rest, but that a body in motion will always keep that motion or change, that is, the same swiftness and the same direction, unless something happens to hinder it. (M. Leibniz, ibid.)
'Everyone clearly sees that this atom, whether it moves by an innate power, as Democritus and Epicurus would have it, or by a power received from the Creator, will always move in the same line equally and after a uniform manner, without ever turning or going back. Epicurus was laughed at, when he invented the motion of declination; it was a needless supposition, which he wanted in order to get out of the labyrinth of a fatal necessity; and he could give no reason for this new part of his system. It was inconsistent with the clearest notions of our minds: for it is evident that an atom which describes a straight line for the space of two days cannot turn away at the beginning of a third, unless it meets with some obstacle, or has a mind all of a sudden to go out of its road, or contains some spring which begins to play at that very moment. The first of these reasons cannot be admitted in a vacuum. The second is impossible, since an atom has not the faculty of thinking. And the third is likewise impossible in a corpuscle that is a perfect unity. I must make some use of all this.
'VI. Caesar's soul is a being to which unity belongs in a strict sense. The faculty of producing thoughts is a property of its nature (so M. Leibniz), which it has received from God, both as to possession and exercise. If the first thought it produces is a sense of pleasure, there is no reason why the second should not likewise be a sense of pleasure; for when the total cause of an effect remains the same, the effect cannot be altered. Now this soul, at the second moment of its existence, does not receive a new faculty of thinking; it only preserves the faculty it had at the first moment, and it is as independent of the concourse of any other cause at the second moment as it was at the first. It must therefore produce again at the second moment the same thought it had produced just before. If it be objected that it ought to be in a state of change, and that it would not be in such a state, in the case that I have supposed; I answer that its change will be like the change of the atom; for an atom which continually moves in the same line acquires a new situation at every moment, but it is like the preceding situation. A soul may therefore continue in its state of change, if it does but produce a new thought like the preceding.
'But suppose it to be not confined within such narrow bounds; it must be granted at least that its going from one thought to another implies some reason of affinity. If I suppose that in a certain moment the soul of Caesar sees a tree with leaves and blossoms, I can conceive that it does immediately desire to see one that has only leaves, and then one that has only blossoms, and that it will thus successively produce several images arising from one another; but one cannot conceive the odd change of thoughts, which have no affinity with, but are even contrary to, one another, and which are so common in men's souls. One cannot apprehend how God could place in the soul of Julius Caesar the principle of what I am going to say. He was without doubt pricked with a pin more than once, when he was sucking; and therefore according to M. Leibniz's hypothesis which I am here considering, his soul must have produced in itself a sense of pain immediately after the pleasant sensations of the sweetness of the milk, which it had enjoyed for the space of two or three minutes. By what springs was it determined to interrupt its pleasures and to give itself all of a sudden a sense of pain, without receiving any intimation of preparing itself to change, and without any new alteration in its substance? If you run over the life of that Roman emperor, every page will afford you matter for a stronger objection than this is.
'VII. The thing would be less incomprehensible if it were supposed that the soul of man is not one spirit but rather a multitude of spirits, each of which has its functions, that begin and end precisely as the changes made in a human body require. By virtue of this supposition it should be said that something analogous to a great number of wheels and springs, or of matters that ferment, disposed according to the changes of our machine, awakens or lulls asleep for a certain time the action of each of those spirits. But then the soul of man would be no longer a single substance but an ens per aggregationem, a collection and heap of substances just like all material beings. We are here in quest of a single being, which produces in itself sometimes joy, sometimes pain, etc., and not of many beings, one of which produces hope, another despair, etc.
'In these observations I have merely cleared and unfolded those which M. Leibniz has done me the honour to examine: and now I shall make some reflexions upon his answers.
'VIII. He says (ibid., p. 332) that the law of the change which happens in the substance of the animal transports him from pleasure to pain at the very moment that a solution of continuity is made in his body; because the law of the indivisible substance of that animal is to represent what is done in his body as we experience it, and even to represent in some manner, and with respect to that body, whatever is done in the world. These words are a very good explication of the grounds of this system; they are, as it were, the unfolding and key of it; but at the same time they are the very things at which the objections of those who take this system to be impossible are levelled. The law M. Leibniz speaks of supposes a decree of God, and shows wherein this system agrees with that of occasional causes. Those two systems agree in this point, that there are laws according to which the soul of man is to represent what is done in the body of man, as we experience it. But they disagree as to the manner of executing those laws. The Cartesians say that God executes them; M. Leibniz will have it, that the soul itself does it; which appears to me impossible, because the soul has not the necessary instruments for such an execution. Now however infinite the power and knowledge of God be, he cannot perform with a machine deprived of a certain piece, what requires the concourse of such a piece. He must supply that defect; but then the effect would be produced by him and not by the machine. I shall show that the soul has not the instruments requisite for the divine law we speak of, and in order to do it I shall make use of a comparison.
'Fancy to yourself an animal created by God and designed to sing continually. It will always sing, that is most certain; but if God designs him a certain tablature, he must necessarily either put it before his eyes or imprint it upon his memory or dispose his muscles in such a manner that according to the laws of mechanism one certain note will always come after another, agreeably to the order of the tablature. Without this one cannot apprehend that the animal can always follow the whole set of the notes appointed him by God. Let us apply this to man's soul. M. Leibniz will have it that it has received not only the power of producing thoughts continually, but also the faculty of following always a certain set of thoughts, which answers the continual changes that happen in the machine of the body. This set of thoughts is like the tablature prescribed to the singing animal above mentioned. Can the soul change its perceptions or modifications at every moment according to such a set of thoughts, without knowing the series of the notes, and actually thinking upon them? But experience teaches us that it knows nothing of it. Were it not at least necessary that in default of such a knowledge, there should be in the soul a set of particular instruments, each of which would be a necessary cause of such and such a thought? Must they not be so placed and disposed as to operate precisely one after another, according to the correspondence pre-established between the changes of the body and the thoughts of the soul? but it is most certain that an immaterial simple and