I shall be concerned in this Section with parts of Professor Blanshard's and of Professor Patterson's essays and with the whole of Professor L. J. Russell's.

(A) SUBSTANCE.

      I propose to open the discussion by distinguishing between an "empirical substance" and "substance in the metaphysical sense."

      There is no difficulty in giving clear instances and clear counterinstances of empirical substances. They include such existents as would commonly be described as "things" or "plants" or "animals" or "persons." A stone, an oak-tree, a pig, and a man are instances which everyqone would admit. On the other hand, I suppose that everyone would decline to describe as an empirical substance either

1. a flash of lightning or a twinge of toothache, or
2. such a localised and dated occurrences of redness and hotness as exists when a poker has been held in the fire for a time and is withdrawn.

      By comparing such existents as would unhesitatingly be described as "things" or "plants" or "animals" or "persons," and by contrasting them with such existents as flashes of lightning, twinges of toothache, etc., we could discover a set of properties which might be described as together characteristic of a typical empirical substance. In a similar way we could discover a set of properties which might be described as together characteristic of a typical empirical continuant. We must not assume, however, that every existent will fall neatly into one or other of the two classes thus demarcated. There are marginal cases, e.g., a whirlpool, which have some features of the one and some of the other, and not all the features of either. Again, there may be existents which do not answer very well to either description. What is one to say, e.g., of a man's mind, as distinct from his body (which is a typical living thing of the animal kind), and from the man himself (who is a typical person)?

      Now the notion of substance in the metaphysical sense arises when one begins to philosophise about typical substances in the empirical sense. One feature of any typical empirical substance is the specially close unity between a number of dissimilar contemporary occurrents, so that they together constitute a single total state of that thing or plant or animal or person. Another feature is the specially close unity between certain successive total states, so that they together constitute the history of that empirical substance, with various overlapping subordinate strands (monotonous or variegated) within it. A third feature is the presence of dispositional properties, some invariant and others variable in accordance with dispositional properties of "higher order." (An example of the former would be mass, and of the latter magnetisation, in the case of a bit of iron.)

      The notion of a substance in the metaphysical sense is an attempt to account for these features of substances in the empirical sense. It involves the notion of a peculiar existent, other than the various empirical occurrents which are counted as states of an empirical substance. This is held to be utterly different in kind from each of the latter severally, and from the complex whole composed of them all collectively in their mutual relations. Let us call such a supposed existent a "substratum."

      For each empirical substance there is supposed to be one and only one substratum, and for each different empirical substance a different substratum. The substratum corresponding to a given empirical substance is supposed

1. to unify various contemporary empirical occurrents into a single total state of that empirical substance,
2. to unify successive total states of it into the total history of it; and
3. to carry its dispositional properties.

      Now I think that the word "substance," in the metaphysical sense, has been used in two different ways by philosophers. Sometimes it denotes a substratum, considered apart from the empirical occurrents which inhere in it and constitute the states (contemporary or successive) of an empirical substance. At other times it denotes the complex whole, consisting of a substratum together with the empirical occurrents which inhere in it, considered as organised by the relation of inherence. For anyone who accepts the theory and uses his terms in the former way, a substance in the metaphysical sense is one constituent, of a unique kind, in a substance in the empirical sense. For one who uses his terms in the latter way, "substance in the empirical sense" and "substance in the metaphysical sense" coincide in extension. Any empirical substance is so constituted as to be a substance in the metaphysical sense, i.e., a unified whole, consisting of a substratum and the occurrents which inhere in it, organised by this relation of inherence.

      Now so far as concerns the application of this metaphysical theory to such empirical substances as stones, trees, pigs, etc., I should agree with what I take to be Professor Blanshard's view of it. The theory is hardly worth discussing in that connexion. And that is because there is nothing to be said about the alleged substratum except either

1. to reiterate the properties which constitute the definition of the term "substratum," or
2. to talk about the particular occurrents and the particular dispositional properties which belong either
   1. to the substratum of a particular empirical substance (e.g., Bucephalus), or
   2. to the substrata of all members of a species of empirical substances (e.g., horses).

      We must note, however, that the notion of a particular in the empirical sense is wider than that of a substance in that sense; for it covers both empirical substances and empirical occurrents and some existents which we might hesitate to classify as either, e.g., a stone, a flash of lightning, and a whirlpool. Now it might be asked whether we are not brought back to the notion of something like a substratum when we reflect on the nature of empirical occurrents. Certainly we have to distinguish in the case of any empirical occurrent (e.g., a flash of lightning) the following two aspects. One of them is the completely determinate, but none the less universal, characteristics, of which it is an instantiation or manifestation, e.g., a certain absolutely determinate shade of blueness, and a certain absolutely determinate shape, extension, and duration. The other is the particularity of this instantiation of those absolutely determinate universals, as contrasted with other actual or possible instantiations of precisely the same determinate universals either simultaneously at other locations or successively at the same location with an interval of time between.

      Now there certainly is a temptation to deal with these two essential and correlated aspects of any empirical occurrent in somewhat the way in which the substratum-theory treats empirical substances. An emptrical occurrent is then thought of in one or other of the two follow ing ways.

1. As consisting of
   1. a short-lived substratum, in which certain determinate qualities inhere so long as it lasts, and which stands while it lasts in determinate spatial relations to other such substrata which are contemporary with it; and
   2. the determinate but universal qualities which inhere in it. (The name "occurrent particular," in the metaphysical sense, might then be given either to such a short-lived substratum considered in abstraction from the qualities which inhere in it; or to the empirical occurrent, considered as consisting of such a substratum together with the qualities which it "supports" and thus unifies.)
2. The other alternative is to take seriously the existence of Absolute Space, as a kind of single persistent substratum, and to think of any empirical occurrent as a region of Absolute Space, of determinate shape, size, and location, pervaded and thus marked out from the rest by certain determinate qualities for a certain period from a certain date.

      I know that all that I have been saying about empirical occurrents must be as familiar to Professor Blanshard as it is to me; and I find the two alternatives, which I have tried to formulate, as unilluminating as he no doubt does. But I have nothing positive of my own to offer, and I must content myself with the following platitudes.

      (1) It is not worth while to get rid of substrata in connexion with empirical substances, if they have to be re-introduced in connexion with empirical occurrents.

      (2) The alternative which presupposes Absolute Space, as a kind of materia prima for all empirical occurrents, seems unfitted to deal with mental occurrents. Can one plausibly (or even intelligibly) allege that, e.g., an experience of anticipating with apprehension a forthcoming visit to one's dentist consists of a certain region of Absolute Space qualified for a certain period by apprehensiveness? The late Professor Alexander was capable of saying such things, but the stomachs of most of us are not strong enough to swallow and digest them.

      (3) In the case of those empirical substances which are persons, we cannot perhaps dismiss the substratum theory so cavalierly as in the case of non-living bodies and plants and non-rational animals. To be a person involves being aware of one's own unity, as contrasted with one's various contemporary experiences, and being aware of one's own identity throughout the sequence of one's successive experiences. Now it has been argued that this is unintelligible except on the hypothesis of a Pure Ego (which would seem to be a substratum of a very special kind), and on the hypothesis that each person is acquainted with his own Pure Ego as well as with his own experiences. In that case he would presumably be intuitively aware of the relation of "supporting," in which the former stands to the latter. And, having acquired the notion of that relation in this way, he might proceed to apply it Justifiably or unjustifiably) to empirical substances other than persons, e.g., to animals, plants, stones, etc. I do not say that I find these contentions convincing. But no account of the unity of a person, with which I am acquainted, seems to me very convincing. So I think that they deserve to be taken seriously, and to be met on their own ground and not just waived aside under the pretext of the general futility of the notion of a substratum. I do not suppose that Professor Blanshard would seriously disagree with this.

(B) THING AND PROCESS.

      Under this heading I shall be concerned mainly with one part of Professor Russell's paper. But before doing so I will comment very briefly on some remarks by Professor Patterson on this topic.

      Professor Patterson says that the phrase "absolute process" conveys nothing intelligible to him. In so far as that is the case, it is not worth while to argue with him about it. But he proceeds to develop some consequences which he thinks would follow from certain statements of mine about processes. I strongly suspect that there is a misunderstanding here, and I will try to clear it up.

      Professor Patterson ascribes to me the opinion that a process is always composed of shorter successive phases which partially overlap. The consequences which he develops seem to be derived from this alleged partial overlapping of successive phases.

      Now, so far as I am aware, I spoke of partial overlapping only in reference to the doctrine of the Specious Present. I thought that the combination of discreteness and continuity, which seems to be involved in the facts

1. that what is speciously present to a person at any moment stretches back for a short period from that moment, with the degree of presentedness tailing off from the later to the earlier extremity of it, and
2. that nevertheless his experience does not come in successive "jerks" or "pulses;" could best be represented as follows.

      I turn now to Professor Russell's essay. This seems to me to give in every instance an extremely fair and adequate summary of what I have written in my Examination of McTaggart's Philosophy on the present subject. I will therefore confine myself to a few reflexions which I have been led to make on re-reading those sections in the light of his comments.

(1) The beginning-to-exist and ceasing-to-exist of a Thing.

      I said that I found no particular difficulty in the notions of beginning-to-exist and ceasing-to-exist, when applied to a thing which is admittedly composed of other things, which enter at a certain time into certain characteristic intimate inter-relations, remain in them for a period, and then gradually or suddenly cease to be inter-related in that particular intimate way. A typical example is provided by an artificial thing, such as a table or a clock. The account would cover also such natural things as, e.g., a crystal of rock-salt. It would need to be considerably elaborated and modified to deal with the case of a living organism, such as an oak-tree or a cat; but I do not think that any fundamental change of principle would be required.

      I said, on the other hand, that I found it difficult to make intelligible to myself the notion of a simple thing beginning or ceasing to exist. For that purpose I meant by a "simple thing" one that does not consist of other things of various kinds inter-related in a characteristic intimate way for a longer or shorter period.

      It seems to me now that this needs more careful consideration. Let us confine our attention to material things, in a fairly wide sense. We ought, I now think, to begin by distinguishing the following two cases, viz.,

1. a thing which is extended but continuous and completely homogeneous, and
2. a thing which is supposed to be literally punctiform, having position but no extension.

      Now anything that is extended is, in a sense, composite, even if it be completely continuous and homogeneous. It is therefore in principle capable of ceasing to exist through the separation of smaller extended parts (all qualitatively exactly like itself and like each other), which were formerly adjoined so that their volumes together exactly made up its volume. It is also in principle capable of coming into existence through the coalescence of such things of the same kind, which were formerly separated. No doubt the old-fashioned "billiard-ball" atom was held to be "indivisible." But that was a contingent physical fact about it, or simply a matter of definition.

      I think that a difficulty might still be felt about the notion of a completely continuous homogeneous extended thing breaking up spontaneously into parts. For it would, so to speak, have no "natural joints." If it is to break up, it must do so in a certain definite way. And, if it is to break up spontaneously, it is difficult to see why it should do so in any one rather than in any other of the innumerable ways which are geometrically possible. But, if it were to break up as a result of external forces, the configuration of these would no doubt determine the particular way in which it would do so.

      Moreover, it seems to me that one could also conceive of another manner in which such a thing could cease to exist or come into existence. This would be by something analogous to evaporation or condensation, as those processes appear at the level of unsophisticated common-sense, and not as a person familiar with molecular theories would regard them. What I have in mind is this. A billiard-ball atom, e.g., might gradually become smaller and smaller without limit through the literal annihilation of one layer after another from circumference to centre, and not through a mere change of state without annihilation of stuff as when water changes gradually from the liquid to the gaseous state. Similarly, one could imagine such a thing coming into existence. A billiard-ball atom, e.g., might gradually grow from nothing to its limiting size through the literal generation of one layer after another from centre to circumference, and not through a mere change of state without generation of stuff as when water changes gradually from the gaseous to the liquid state.

      It might be remarked, however, that reflective persons have not rested content with the prima facie appearances in the case of evaporation and condensation or in that of chemical generation and destruction. This might suggest that there is some intellectual difficulty in the notions of the literal annihilation or generation of a homogeneous continuous extended thing. Against this it might be said that the sophisticated interpretation (in terms, e.g., of the physical theory of molecules and the chemical theory of atoms) synthesises a large number of very pervasive facts, which reveal themselves only at a fairly advanced stage of observation and experiment, and that it is these facts which originally suggested it and are the only evidence for it. Yet, on the other hand, it should be noted that atomic theories were put forward long before these facts were known or suspected. So one may be inclined to think that they were motivated by difficulties felt in the notions of the literal annihilation or generation of a homogeneous continuous extended thing.

      However that may be, I conclude that I must greatly tone down my remarks about the a priori objection to the coming-into-existence or the ceasing-to-exist of a simple thing, when "simple" is taken to include as one possibility being extended but completely continuous and homogeneous.

      I pass therefore to the case of a thing which is "simple," in the sense of being literally punctiform, e.g., a Boscovichian atom. Obviously it could not come into existence either by the coalescence of pre-existing smaller things of the same kind, which were formerly separated, or by gradual generation of fresh layers about a centre. Nor could it cease to exist by the converse of either of those processes. Now, in order to count as a "thing," such an existent would have to have some dispositional properties. It would be possible to distinguish, at any rate verbally, between the following two cases, viz.,

1. beginning to exist at t, and
2. having existed before t with nothing but unactualised dispositions, and some or all of these being actualised for the first time at t.

      I would sum up about things which are simple, in the sense of punctiform, as follows. Since such a thing could not begin or cease to exist in any of the ways which are familiar and seem intelligible to us, there is a temptation (into which I have tended to fall) to say that it is unintelligible that it should begin or cease to exist. A person who wished to maintain that proposition, and yet was presented with prima facie instances to the contrary, could always verbally save his case by adopting the expedient mentioned in the preceding paragraph. But whether anything significant corresponds to the distinction which he draws in words, might fairly be doubted.

(2) "Absolute Process."

      Professor Russell says that I have given an account of "processes" which seems to suggest that a process could not be perceived by more than one person, and to make it doubtful whether there could be unperceived processes. If anything that I wrote gives that impression, it is to that extent misleading, for I certainly had no intention of denying the possibility either of a process being perceived by more than one person, or of there being unperceived or even imperceptible processes.

      I suspect that the misunderstanding may have arisen in the following way.

1. I gave the instance of a sound (in the sense of something which has the sensible features of pitch, loudness, etc., as actually experienced) as illustrating in certain respects what I had in mind when I talked of an "absolute process."
2. I then remarked that it might be, and has been, doubted whether a sound, in that sense of the word, could exist except as a factor in an auditory experience of some one particular individual on some one particular occasion.

      The example of a sound (in the sense in question) is useful only in so far as it illustrates the notion of a process which is not at all obviously a state of invariance or a state of change in the qualities or the relation ships of any "thing." Its prima facie defect as an illustration (apart from the one just mentioned, which led Professor Russell astray) is that we all believe a sound to be causally dependent on processes in what we commonly take to be things, viz., the outer air, the auditory nerves, the brain, and so on. But I do not think that this is really a defect. It was no part of my notion of an "absolute process" that it should not be causally dependent on processes which are themselves states of invariance or of change in the qualities or relationships of things. (The use of the word "absolute" may have been misleading here.) The essential point was that it should not itself be a state of invariance or of change in the qualities or relationships of any thing. I admit, of course, that the example, even if it were certainly an instance of an absolute process, could not be used in support of the alleged possibility of dispensing with the notion of things and describing all the facts in terms of absolute processes.

      I fully agree with Professor Russell that there is no hope of realising the latter possibility unless we are prepared to admit the existence of absolute processes which are not in fact perceived, and of ones which are in principle imperceptible. But I see no objection to the possibility of such absolute processes.

      When physicists talk of electro-magnetic fields, of trains of electromagnetic waves, and so on, and when they at the same time disclaim all belief in the old-fashioned substantial ether, what can they have in mind except what I have called "absolute processes"? If that be so, it would reinforce what Professor Russell contends as to the possibility and the necessity of ascribing dispositional properties to absolute processes. Obviously a wireless beam, e.g., carrying a pattern of modulations, has plenty of dispositional properties. And, if there be no substantial ether, these must be ascribed to the beam itself, and not to any "thing," of which the beam is a state.

(C) CAUSATION.

      A part of Professor Blanshard's essay is concerned with questions about causation which are mainly epistemological, and a part of Professor Russell's paper with questions about causation which are mainly analytical. I will take the two in turn.

(I) Epistemological Questions.

      What Professor Blanshard has to say on this topic occurs in his discussion of the power of "intuiting necessary connexions," which is one of the functions that has been ascribed to "Reason." I would make the following comments.

1. I do not think that there would be any inconsistency in combining the following two views: --
   1. That we have a priori knowledge of certain general principles about causation, and
   2. that we have no a priori knowledge of any particular causal law.
2. Again, there seems to me to be no inconsistency, at any rate at the first move, in combining the following two views: --
   1. That the notion of causation is (or contains as an essential ingredient) a concept which is not empirical, and
   2. that our knowledge of any particular causal law is empirical.

      Suppose, however, that we then raise the question: -- Assuming that the notion of causation is not wholly empirical, how do we come to have it? Suppose, further, that we are not content to say that the capacity and the tendency to formulate causal judgments is innate, though requiring certain specific kinds of experience to activate and direct it. Then we might be forced to conclude that we must have derived the notion (or at any rate the non-empirical ingredient in it) from being acquainted, in some non-sensuous and non-introspective way, with instances of it. And that would seem to be equivalent to saying that in some cases we must have had a priori knowledge of particular instances of causal connexion.

      (iii) Professor Blanshard gives examples where he thinks it plausible to hold that we do in fact see, by merely reflecting on the natures of the events in question, that any event of a certain kind has at least an intrinsic tendency to be accompanied or immediately followed by an event of a certain other kind in a certain relation to it. One alleged example is that an experience of expecting to suffer severe pain has an intrinsic tendency to be accompanied or immediately followed in the same conscious being by an experience of fear.

      I admit the prima facie plausibility of such examples- but I suspect that the apparent synthetic a priori judgment may really be a conflation of one which is a priori but analytic with another which is synthetic but empirical. I think that the word "fear" connotes

1. an experience involving certain feelings associated with certain bodily states (e.g., a "sinking feeling" in the stomach, the feelings associated with a cold sweat, and so on), and
2. a reference to certain kinds of situation (e.g., dangerous, painful, or embarrassing ones).

(2) Analytical Questions.

      Professor Russell very justifiably finds much that is obscure in my remarks about causation in Examination of McTaggart's Philosophy Vol. I Chapter XIII. He tries to lighten the darkness by restating what I may have had in mind in terms of the notions of necessary condition and sufficient condition. I am sure that that is the right course. I have pursued it myself in later writings. I think that the simplest way for me to clear up the matter is to begin by giving some definitions and making some statements based on the contents of Pp. 15 to 18 of the first of my papers entitled, "Hr. von Wright on the Logic of Induction," in Mind, Vol. LIII.

1. P is a sufficient precursor ("S.Pr.") of Q, if from any instant into which a P-event were to enter a Q-event would issue.
2. P is a necessary precursor ("N.Pr.") of Q, if into any instant from which a Q-event were to issue a P-event would have entered.
3. P is a smallest sufficient precursor ("S.S.Pr.") of Q, if
   1. it is a sufficient precursor of Q, and
   2. it is either
      1. a simple characteristic p, or
      2. a conjunctive characteristic p₁&p₂&. . . . p_n, such that, if any of the conjuncts be omitted, what remains is not a sufficient precursor of Q.
4. A contributory precursive condition ("Cy.Pr.Cn.") of Q is any simple characteristic, or any conjunction of such characteristics, which is a conjunct in a S.S.Pr. of Q.
5. P is a smallest necessary precursor ("S.N.Pr.") of Q, if
   1. it is a necessary precursor of Q, and
   2. it is either
      1. a simple characteristic p, or
      2. a disjunctive characteristic p₁or-p₂or-. . . .p_n, such that, if any of the alternants be omitted, what remains is not a necessary precursor of Q.
6. A substitutable precursive requirement ("Sb.Pr.Rq.") of Q is any simple characteristic, or any disjunction of such characteristics, which is an alternant in any S.N.Pr. of Q.
7. If Q has only one S.S.Pr., every conjunct in it may be described as an indispensable contributory precursive condition ("I.Cy.Pr.Cn.") of Q. If, on the other hand, Q has several alternative S.S.Pr's, then any characteristic which is a conjunct in all of them may be so described.

     So much by way of definition. It is important to be clear about the logical relationships of the two notions of necessary precursor and indispensable contributory precursive condition.

      (a) It is logically possible for P to be an I.Cy.Pr.Cn. of Q without being a N.Pr. of Q. For, whether there be only one S.S.Pr. of Q or several alternative S.S.Pr's of Q. it remains logically possible that there should be cases in which a Q-event issues from an instant into which no S.S.Pr. of Q has entered. Now, in order for P to be a N.Pr. of Q, a P-event would have to enter into every instant from which a Q-event issues. So a P-event would have to enter inter alia into those instants (if any) from which a Q-event issues without any S.S.Pr. of Q having entered. But, in order for P to be an I.Cy.Pr.Cn. of Q. it has only to be a conjunct in every S.S.Pr. of Q. Obviously that does not guarantee the entry of a P-event into those instants from which a Q-event issues without any S.S.Pr. of Q having entered. Since it is logically possible that there should be such instants, it is logically possible for P to be an I.Cy.Pr.Cn. of Q without being a N.Pr. of Q.

      (b) This possibility would be ruled out, if and only if we were to assume that in every case in which a Q-event issues from an instant there is some S.S.Pr. of Q. This might be called the Postulate of Smallest Sufficient Precursors. On that assumption any I.Cy.Pr.Cn. of Q must be a N.Pr.of Q.

      (c) It is logically necessary that a conjunction of all the I.Cy.Pr.Cn's of Q should be a S.Pr. of Q. But it is not logically necessary that a conjunction of all the N.Pr's of Q should be a S.Pr. of Q. The latter proposition would, however, follow from the Postulate of Smallest Suflicient Precursors.

      Now it is certain that neither the above distinctions, nor consequently the logical relations between the notions distinguished, were clearly before my mind when I wrote the chapter on Causation. In terms of them I will make the following comments on certain things which I wrote there.

      (i) I think that what is generally understood by the phrase "total cause of such and such a change issuing from an instant t" is a S.S.Pr. for changes of that kind. Therefore the most obvious interpretation of the sentence: "All changes of such and such a kind are caused" would be that in every case where a change of that kind issues from an instant there is a S.S.Pr. for it. That would be quite consistent with holding that there is a plurality of alternative S.S.Pr's for changes of that kind; that in some cases one is present and in other cases another; and that perhaps in some cases several of them are present together. The most obvious interpretation of the sentence: "All changes whatever are caused" would be a generalization of the above statement about all changes of such and such a kind. It would in fact be the Postulate of Smallest Suflicient Precursors.

      (ii) Any reader might be excused for thinking that it was this proposition which I claimed to find self-evident when I wrote (Examination Vol. I P. 232) "Every change has a cause," and said that this was to me evidently true. But in fact I did not, and do not, find it self-evident that for every case in which a change of any kind issues from an instant there must be a S.S.Pr. for a change of that kind issuing from that

      If the reader should continue until he reaches the discussion of voluntary decision on P. 238 of the volume in question, he will find that what I there claim to be self-evident would be expressed (at any rate to a first approximation) by the following sentence: -- "In every case in which a change of any kind issues from an instant there must be a change entering into that instant, such that a change of the former kind would not have issued unless one of the latter kind had entered." Now this, as Professor Russell rightly points out, is an assertion about necessary precursors, and not about sufficient precursors.

      The above statement needs a certain amount of elucidatory comment, in view of the fact (which I did not recognise at the time) that a N.Pr. need not be simple, and that the S.N.Pr. for a given kind of change may be a disjunction. (I owe the recognition of this to Professor von Wright.)

      It might be that an event of the Q-kind would not issue from any instant unless an event of a certain kind P₁-or-P₂ should have entered into that instant, but that in some cases the entering event is of the kind P₁ and there is none of the kind P₂, that in others it is of the kind P₂ and there is none of the kind P₁, and that in yet others perhaps there is either a single entering event of the two kinds or two entering events one of each kind.

      What I claimed to find self-evident might therefore be re-stated as follows. The issuing of an event of any given kind (say Q) from any instant must be preceded by the entry into that instant of an event which is either

1. of a certain one kind (the same in all such cases), or
2. of one or another of a certain limited number of alternative kinds (in some such cases of one, and in other such cases of another, of these alternative kinds.)

      I think I may say of this proposition the following two things.

1. The contradictory of it is certainly not self-contradictory.
2. When I reflect on the contradictory of it, and try to consider "what such a state of affairs would be like," I find it almost impossible to think that it could be true.

      (iii) I should not now be inclined to attach much, if any, weight to the proposition which I asserted, at the bottom of P. 233 of Vol. I of Examination, to be self-evident. This to the effect that a given change issuing from a given instant cannot have "more than one total cause." I should now identify "a total cause" of a particular change with any S.S.Pr. of such changes which enters into the instant from which that change issues. If there should be only one S.S.Pr. entering into the instant in question, we can talk of "the total cause" of the change on that occasion. But such a change may have several alternative S.S.Pr's, and it does not seem to me self-evidently impossible that more than one of them should enter into a given instant from which such a change issues. In that case, it seems to me, we must be content to say that the particular change in question has several coexisting total causes, and therefore that there is nothing that can be called "the total cause" of it. I should describe such a change as "over-determined." It would be easy to produce quite plausible prima facie instances of over-determination.