1 “(My) system has besides this advantage, to conserve in all its rigor and generality this great principle of physics, that a body never undergoes a change in its motion, except by another moving body that pushes it… . This Law has been violated up to this time by all those who have admitted Souls or immaterial principles, including even all the Cartesians. The Democriteans, Hobbes and some other pure materialists, who have rejected all immaterial substance, having alone conserved this law up till now, believed they had found in this a ground for insulting other philosophers, as if they were maintaining in this respect an extremely unreasonable opinion. But the ground of their triumph was only apparent and ad hominem; and far from being able to aid them, it serves to bring them down. And now that their illusion has been discovered, and their advantage turned against them, it seems one can say that this is the first time that the best philosophy is shown to be also the most agreeable with reason in everything, nothing being left with which to oppose it.”

“Considerations on the Principles of Life, and on Plastic Natures …,” Leibniz, Die Philosophischen Schriften, ed. Gerhardt, C. J. (Berlin, 1875-90), vol. 6, p. 541Google Scholar; cf. Leibniz, : Philosophical Papers and Letters, ed. Loemker, Leroy (Dordrecht-Holland, 1969 (2nd ed.) ), No. 61Google Scholar. (Translations in this paper are my own unless otherwise indicated; however, the translations of Leibniz have often been influenced by Loemker's versions. In most cases I give references both to an original language edition and to a convenient translation of the work. In citing Loemker's translations I usually use selection numbers rather than page numbers, which are different in the first and second editions.) In the same paper Leibniz suggests that Descartes himself would have embraced parallelism, had he recognized that motion is conserved as a vector, and not as a scalar quantity.

2 Principles of Philosophy, I, 39, 41 (The Philosophical Works of Descartes, trans. Haldane, E. S. and Ross, G. R. T. Dover, 1934 (2nd ed.), vol. I, 234–35)Google Scholar; Reply to (the twelfth part of) Objections III, ibid., vol. II, 75. Hereafter this translation is abbreviated H&R.

3 Theodicy, “Essay on the Goodness of God,” Part I, § 50 (Gerhardt, vol. VI, 130; ed. Diogenes Allen, Bobbs-Merrill, 1966, p. 51).

4 One point Leibniz is making in this context is that there is no “liberty of indifference”- i.e. no strictly arbitrary act. But the possibility of sub-conscious causes also is relevant to cases where one believes, for example, that the whole explanation of one's having done A is to be found in one's conscious desire for X.

5 It isn't clear whether the example is supposed to suggest that desires might just happen to coincide with bodily movements, or rather that they might be effects rather than causes of conditions sufficient for the movement “desired.” In fact Leibniz's own official view is somewhat closer to the first position: there is no causal connection between mental and physical states, but God sees to it that there is an appropriate “harmony” between the two series. Incidentally, Leibniz's mention of a “vivid internal feeling” seems to go beyond Descartes’ relatively bland characterizations of the apprehension of freedom.

6 See, e.g. the “fourthly” clause of the Replies to Objections II, especially H&R II, 42.

7 Leibniz remarks that what Descartes “adds” to the argument in the Meditations (presumably this would include the reference to God-see below) will be examined “in its proper place.” As far as I know he does not carry out this promise.

8 On Article 8 of Part I, Gerhardt IV, 359; cf. Loemker No. 42.

9 Ibid.

10 Oeuvres de Descartes, ed. C. Adam and P. Tannery, VIII Part I, 7.

11 See, besides Meditations ii and vi, his reply to the first part of Objections II, H&R II, 30ff.

12 Adam and Tannery, IX, 000; H&R I, 190.

13 See, for example, “Observations on … Descartes Principles,” ad. Art. 43, 45, 46 (Loemker, No. 42); Courturat, L. La logique de Leibniz, Paris: Alcan, 1901, pp. 100Google Scholar n. 2 and 203 n. 2. I have discussed some related aspects of the contrast between Leibnizian, and Cartesian, epistemologies in “Leibniz and Locke on ‘First Truths’,” Journal of the History of Ideas, vol. XXVIII (1967), 347–66.Google Scholar

14 Nagel, Thomas “Armstrong on the Mind,” Philosophical Review, LXXIX, July, 1970, 402.Google Scholar Nagel comes closer to the Leibnizian viewpoint in his previous sentence, which reads: “The real issue is whether one can know that one has conceived such a thing [as mental states existing apart from body) …. ” From this sentence to the next Nagel seems to move without comment from “Can we know we have conceived such a thing?” to “Can we know we cannot conceived such a thing?” For Leibniz a negative answer to the first question is sufficient to “defeat the Cartesian argument.” See also Leibniz's letter to Malebranche, June 22/July 2, 1679, (Gerhardt I, 332; Loemaker No. 22, ii):

The distinction of the soul and the body is not yet completely proved. For since you admit that we do not conceive the nature of thought [ce que c'est la pensee] distinctly. it is not sufficient that we doubt the existence of extension. (that is, of that which we conceive distinctly) without being able to doubt thought; that, I say, is not sufficient to draw a conclusion about how far the distinction goes between what is extended and what thinks, for one may say that it is perhaps our ignorance that distinguishes them, and that thought includes extension in a way that is unknown to us.

15 See Kripke, Saul “Naming and Necessity,” in Semantics of Natural Languages, ed. Davidson, D. and Harman, G. (Dordrecht-Holland, 1971), 253–355.Google Scholar

16 P. Malebranche, Entretiens sur Ia metaphysique et sur la religion, I, i. (I am grateful to Willis Doney for providing this reference.) Although there can be little question that this is the passage Leibniz draws on, his statement of the argument has a rather different character than Malebranche's own. Malebranche in effect argues that it is just evident that thoughts and mental states generally are one thing, and that attributes of matter something else.

17 Gerhardt, VI, 587; Loemker, No. 64

18 Gerhardt, VI, 586.

19 In recent controversy a point that has particularly been discussed is whether thoughts can be said to have spatial location (and if not, what the consequences of this would be for the materialist thesis). See for instance Schaffer's, Jerome well- known paper “Could Mental Events be Brain Processes?”, Journal of Philosophy 58 (1961), 813–22.CrossRefGoogle Scholar However, the argument that Leibniz here repudiates could be interpreted differently, as depending on the principle of non-contradiction rather than Leibniz's Law, viz.: “if anything is a brain state it is immeasurable; if anything is a thought it is not measurable; if anything is a brain-state and a thought it is measurable and not measurable; hence no thought is a brain state.” Whatever the importance of this distinction of logic, it does not appear that there is any need for a distinction with respect to the reply or counter-argument-especially since the one Leibniz offers here is in fact used (with some refinements) by Smart and other materialists in reply to Leibniz's Law arguments.

Note, incidently, that Leibniz's treatment of the Cartesian epistemological argument (discussed above) suggests that he wasn't prepared even to entertain the idea that “Leibniz's Law” could yield the non-identity of mind and body via discrepancies in their (respective) “intensional” predicates.

20 Gerhardt, loc. cit.

21 Ibid.

22 In this passage (and elsewhere, for that matter) Leibniz does not enter into subleties about the relation of language to theory.

23 See, for example, “Meditations on Knowledge, Truth, and Ideas,” Gerhardt IV, 422-26; Loemker, No. 33. That this is the correct interpretation of Leibniz's point seems strongly confirmed by the following passage, from a letter to Queen Sophie Charlotte (Wiener, P. ed. and trans., Leibniz Selections (New York: Scribners, 1951), 355Google Scholar:

We use the external senses as … a blind man does a stick, and they make us know their particular objects, which are colors, sounds, odors, flavors, and the qualities of touch. But they do not make us know what these sensible qualities are or in what they consist. For example, whether red is the revolving of certain small globules which it is claimed cause light; whether heat is the whirling of. a very fine dust; whether sound is made in the air as circles in the water when a stone is thrown into it, as certain philosophers claim; this is what we do not see. And we could not even understand how this revolving, these whirlings and these circles, if they should be real, should cause exactly these perceptions which we have of red, of heat, of noise.

This letter is cited by Ishiguro, Hidé in “Leibniz and the Ideas of Sensible Qualities” (in Reason and Reality, Royal Institute of Philosophy lectures, vol. V, 1970-71Google Scholar, ed. G. Vesey [London: Macmillan, 1972), pp. 49–63). lshiguro's entire discussion bears directly on the points I discuss in the next few pages; I regret that I did not know of her interesting essay earlier. It would appear from this and other passages quoted by lshiguro that there is an unresolved ambivalence in Leibniz's views on the reference of such terms as “heat” and “red,” which may help explain the difficulties in his reply to Malebranche. In brief, he seems unable to decide whether heat, for example, should be identified with the cause of a particular sort of experience or with the experience itself.

24 See, e.g. “Meditations of Knowledge, Truth, and Ideas,” loc. cit. Cf. New Essays on Human Understanding, III, iv, §.§ 4, 5, 6, 7 (ed. Langley, A. G. (New York; Open Court, 1916; 2nd ed., p. 319Google Scholar; Leibniz, Samtliche Schriften und Briefe,ed. Deutsche Akademie der Wissenschaften zu Berlin, Reihe 6, Bd. 6 (Berlin: Akademie-Verlag, 1962), p. 296.Google Scholar

25 New Essays, II, ii.. § 1 (Langley, 120-21; Deutsche Akademie, 120); cf. § 4. In New Essays, II, xxxix, §§ 9, 10, Leibniz characterizes “confusion” as “the lack (defaut) of the analysis of the notion that one has” (Langley, 270; Deutsche Akademie, 258). Leibniz's discussion of § 13 suggests that an idea is distinct only if it enables one to discover the nature and properties of its object.

26 Cf. New Essays, Preface (Langley, 48; Deutsche Akademie, 54; also Loemker, 2nd ed., p. 339.)

27 New Essays, II, viii, § 15 (Langley, 133; D.A. 131). Leibniz likes to claim that, applied to perception, this principle (with its implication of the internal complexity of a sensation) yields the conclusion that the correlation of a particular mind-state with a particular state of matter is not arbitrary or inexplicable, as Descartes thought. (See, e.g., New Essays, Preface [Langley, 50; D.A., 56]). There may be reason to question the consistency of this talk of perceptions in terms of external causes with Leibniz's parallelist claim that mental states have only mental causes. In fact, Leibniz sometimes distinguishes the causality of sensation (which is said to “be regulated on the body,” and to “express the laws of motion according to the order of efficient causes”) from that of the higher mental states (said to be caused by the will). See esp. Gerhardt IV, 591.

28 Actually the matter is more complicated than this. Since Leibniz rejects conventionalism, he holds that we might assume that a definition or mark is distinct in the sense of uniquely individuating a given species when it is not. In fact, three or four separate questions are run together rather confusingly in Leibniz's various pronouncements on what it is to·have a distinct idea of X:

(1) Can we state a mark of X's, as opposed to just “knowing them when we see them”?

(2) Can we give a characterization of X's which (in reality) uniquely picks out X's?

(3) Do we thoroughly understand the “nature or causes” of X-ness?

(4) Is our explicit knowledge of the properties of X's sufficient to allow (potentially) a full deduction of their properties, qua X's?

29 lshiguro (op. cit.) presents a number of details relating to this point. However, at least part of the reason for the lack of a sharp distinction in Leibniz's philosophy between causally explaining, and analyzing a concept must be found in his uncertainty about whether e.g. heat is to be identified with a physical state or with the perception caused by the state (see n. 23 above). If one vaguely assumes that it is both at once, then a physical theory of heat may seem to be both an analysis of the nature of heat, and a causal account of heat.

30 Monadology § 17 (Gerhardt, VI, 609; Loemker No. 67).

31 Ibid. Cf. Essays, Preface (Langley, 61-2; P.A. 66-7).

32 Louis E. Loeb has shown, in an interesting unpublished paper (“Leibniz's Conception of Miracles,” 1972) that Leibniz's criterion for determining whether a phenomenon is natural (as opposed to miraculous) is whether or not it is explainable mechanistically.

33 “A New System of the Nature and Communication of Substances,”paragraph 11 (Gerhardt VI, 482; Loemker No. 47, pt. 1).

34 Ibid., paragraph 3; cf. Russell, Bertrand A Critical Expositon of the Philosophy of Leibniz(London, G. Allen & Unwin, 1937 (2nd ed.]), ch. 8; also Gerhardt IV, 559–60Google Scholar (Loemker, 2nd ed., 577-8):

But besides the principles which establish Monads, of which composites are only the results, internal experience refutes the Epicurean doctrine: the consciousness that is in us of this me which apperceives the things that happen in the body; and the fact that perception cannot be explained by figures and motions establishes the other half of my hypothesis, and makes us recognize in ourselves an indivisible substance, that must itself be the source of its phenomena. so that, following this second half of my hypothesis, everything takes place in the soul as if there were no body, just as according to the first half, everything occurs in the body as if there were no soul.

35 But in the Conversation of Philarète and Ariste, cited above, leibniz himself suggests that one should not accept as axiomatic the exhaustiveness of the mind-matter dichotomy.

36 Gerhardt, II, 112; Loemker, No. 36, pt. ll-2nd ed., p. 339.

37 Monadology, § 16.

38 Sometimes, however, Leibniz identifies our inner experience of “activity” as the key to our knowledge of ourselves as immaterial substances. See for instance Loemker, No. 2, n. 17; No. 64. As he himself seems to point out in the latter work (“Philarète and Ariste”), however, this identification appears to go through only on the Cartesian conception of matter as essentially inert—a conception that Leibniz rejects.

39 Cf. Kant, I. Selected Pre-Critical Writings, ed. Kerferd, G. B. and Walford, D. E. (Manchester University Press, 1968), III.Google Scholar

40 E.g., Bennett, Jonathan “The Simplicity of the Soul,” in The First Critique, ed. Penelhum, T. and Mcintosh, J. J. (Wadsworth, 1969), pp. 109–122Google Scholar, reprinted from The journal of Philosophy (1967), pp. 648–660. (But Professor Bennett has indicated in correspondence that he too now regards the argument as more Leibnizian than Cartesian.)

41 Critique of Pure Reason, trans. Norman Kemp Smith (London: Macmillan, 19), A 351 (Second Paralogism: of Simplicity).

42 A351-52. In citing Kant I follow the Kemp Smith translation because it is generally reliable and widely used. The translation of this passage does contain one error affecting the sense: “that a thought should inhere in what is essentially composite” (next-to-last sentence) should read, “that a thought should inhere in the composite as composite” (the locution is the same as in the first sentence).

43 A 354.

44 A 355.

45 Kant, however, holds that the self is not presentable to sense and hence unknowable to us. So his rejection of the :Leibnizian argument is not conceived as opening the way for a naturalistic account of thought or subjectivity.

46 A 363-64. But perhaps Kant offers this fable only as a reductio of (what he takes to be) the rational psychologist's definition of ‘identical person’.

47 Chisholm, Roderick Theory of Knowledge(Englewood Cliffs, N.J.: Prentice-Hall, 1966), p. 102.Google Scholar Compare Nagel, Thomas “Physicalism,” Philosophical Review 74 (1965), pp. 353ffCrossRefGoogle Scholar (a suggestive discussion of a closely related point).

48 I wish to thank Fabrizio Mondadori and Stephen Barker for helpful comments.