The ship W, then, cannot be a substance (genuine whole), for it has different parts on different days. But then where, Chisholm asks, "are we to find an instance of our concept of substance?" (Ibid.) He finds the answer by considering "the relation that W bears at any time to the aggregate, or heap, of its parts that exist at the time (say, the relation it bears on the third day to the aggregate AYZ)." These "aggregates" do indeed hold their parts throughout the time indicated. Indeed Chisholm's view is that the aggregate ABCXYZ and all of its proper parts persist from Monday through Thursday. Thus: "The aggregate XYZ didn't come into being on Thursday. It was there all along--but prior to Thursday it wasn't a ship (it didn't have a ship as a mode) and it was more widely scattered than it was on Thursday." (Ibid.) Being 'in' an aggregate is not a matter of spatial location. Y and Z could have been in Siberia and South Africa respectively on Monday and still be a 'part' of the aggregate XYZ existing on Monday and constituting the ship on Thursday. This point is of considerable importance in understanding what Chisholm takes an aggregate to be. Clearly it is not a heap in any ordinary sense of that word, and the only condition which A must meet to be in the aggregate AB etc. is that it exist. Aggregates are not eternal beings, and they do cease to exist whenever one or more members do.

But clearly also, for ABC to be an aggregate is not enough for it to make ("constitute") a ship (table, tree, etc.). As Chisholm says, "XYZ...was there all along--but prior to Thursday it wasn't a ship." The aggregate (substance) can exist without being (constituting) a ship, but a ship cannot exist except as a mode of an aggregate. In sum: "If the ship W is a mode and not a substance, we need not hesitate to say that it changes its parts from one day to the next. But the various aggregates that the situation involves never change their parts. For they are substances." (Ibid.)

Misgivings:

Chisholm's account of modes (physical things) and substances (true wholes=aggregates) obviously differs greatly from the accounts of mode and substance familiar from the Western philosophical tradition. Traditionally, modes could not transfer from substance to substance, and the unity of a substance was much more than that found in an aggregrate or 'set'. But there are problems facing his account that are more serious than non-conformity with tradition. These may suggest that we should look for another way of dealing with the problem that is actually driving Chisholm: the well-know puzzles about the persistence of physical objects such as Theseus' ship.

1. The 'Reformulations': To begin with, consider his attempted 'reformulations' of ME and of our claims about the car and its tires. ME initially seemed to be about such things as the car. That is what made it both interesting and shocking. Now we learn that it is only about aggregates. But how did we learn that it is only about aggregates? So far as I can tell, the only reason for saying that it is really about aggregates is that it comes out true when so read, and can be worked into a certain proposed solution to the persistence puzzles. But whether or not it is true was the initial questions, and its truth therefore should not be used to force a reading of its sense. The reformulations given by Chisholm have no more to recommend them than that they resolve the alleged conflict of intuitions.

Accordingly, if the alleged conflict can be resolved in other ways, the reformulations are unnecessary, and perhaps unfounded. One way of resolving it is simply to insist that ME in its general formulation is not intuitively plausible, or even true, and that it becomes or seems so only when we think of wholes from the outset as mere aggregates. The aggregate ABC is not the aggregate DBC, and no longer exists if you 'replace' A in it with D. Indeed, the whole idea of replacing a member of an aggregate with another is nonsense from the outset. If you take A out (destroy it) there is no 'place' left to 'insert' D, for ABC has just ceased to exist. This is, of course, strikingly unlike replacing a part in a physical object: a leg on a table or a tire on a car. ME is intuitively plausible if it is restricted to aggregates and their elements, but then it is no intuition about wholes and parts generally, nor about physical things in particular. This is because, to put it crudely, when some parts of these latter are removed, the other parts often remain in their interrelationships, preserving a continuous context of replacement. They preserve a 'place' for a new part to be inserted, sustaining wholeness and retaining identity.

If this does not happen, the whole ceases to exist and the part removed is thereby shown to be essential to its whole. Replacement either cannot occur or yields a new whole. As Alvin Plantinga observes: "If I replace a tire on my automobile, we think the same automobile persists through the change, acquiring a new part. But if I replace the automobile on my tire, then the whole that contains my tire is not the whole I began with."⁷ I think this is true because we now have in the latter case a different context of replacement, and the context of replacement is what provides the continuing identity of the physical object through suitable changes of its parts and properties.

So we take the position that Chisholm's "conflict of intuitions" is resolved by understanding that ME as a general principle of wholes and parts that would contradict the claims about the tire, etc. is not intuitively plausible, and indeed is not true. The conflict is a genuine contradiction, but it is not a conflict of intutions. A restricted mereological essentialism remains true, however. There are some wholes with parts that are essential to them, and among such wholes are aggregates. The truth of Mereological Essentialism is that there are parts the loss of which destroys the corresponding whole, and such parts are, of course, essential to their wholes.

2. Aggregates Not Sufficient To Constitute Physical Objects: Aggregates do not merely as such constitute a physical object. There must be a modification of the aggregate before it becomes the substance or substrata of an ordinary object. The parts that compose the aggregate must interrelate in specific ways that constitute a ship, table, or house. We emphasized from Chisholm's exposition above that, according to him, the aggregate XYZ existed on Monday through Wednesday--and for all we know long before that--but it did not constitute the ship until Thursday. Something happened to the members of the aggregate XYZ--When? At midnight Wednesday?--that made them into a ship. So Chisholm says that before Thursday the aggregate "was more widely scattered than it was on Wednesday." Now I am not sure that the aggregate is even the sort of thing that can be more or less 'scattered'. I think not. And in any case it was not just more widely scattered, as if before Thursday Z, Y, and X were all one hundred miles apart and on Thursday they were within a radius of a few feet. Being scattered more or less widely is not the point. The point is that before Thursday they were not organized into an entity such as a ship or table. They were not in the pertinent relationships to one another. And this is not a matter of being less "scattered." In fact, it is conceivable that the pieces of the ship or table could be less widely scattered than they are when they actually constitute or make up the ship or table. For example, they could be arranged together to occupy less space, or to encompass less empty space, than they do when they actually constitute a ship or table.

But in any case there is a very great and obvious difference between a mere aggregate, and a group of things that make up a physical object at a time. The aggregate can exist without the object. Chisholm recognizes this. But he has no account of the difference when the aggregate is a physical object, and seems to neglect its importance in the process of aggregates becoming ordinary physical objects, which he explicitly admits they do. Thus, after telling the story about the "very simple table" he says: "And so we have described one possible way of looking upon what happens when, as we would ordinarily put it, a thing such as a table undergoes a change of parts." (OM 71) Our reply must be that he has described no such thing. He has said nothing at all about "what happens when...a thing such as a table undergoes a change of parts." We are presented with a static picture of times at which a thing (the table) has had parts replaced. But if he had ever seriously considered what happens when the table undergoes a change of parts he would, I think, never have given Complete Unbridled Mereological Inessentialism (CUMI) a thought. He would never have considered that this table might have Grand Central Station and his left foot (OM 68), or the number 36 and the property blue (PO 147), as parts, for he would have seen the impossibility or just the senselessness of replacing the stump or the board with Grand Central Station, the property blue, etc., etc.

The possibility of replacement is, however, the heart of the matter, so far as the essentiality or non-essentiality of the pieces of a physical object to that object is concerned. In the usual case the physical object has a very large number of pieces. (The simpleness of Chisholm's preferred cases and models is, we have suggested, prejudicial in favor of his conclusions.) These many pieces have specific relationships to one another, and in many if not most cases the relationships continue to hold between the parts even when one or a fiew of them are lost from the object. If you remove the tire from the car or the leg from an ordinary table, the other parts in the whole remain, at least for a significant time, in their customary relations. Greater lengths of time are a different matter, for the relations between the remaing parts have various dependencies upon the parts that are missing. Everyone knows that the tendency of a car without a tire or a table without a leg is toward total dissolution, though unusual circumstances--e.g. they are in a museum or in storage--can make a difference. The other parts are related in ways that cannot be indefinitely sustained unless the tire or leg is there and the whole can function as intended. A deciduous tree drops its leaves in the fall and replaces them in the spring. But if, for any reason, the leaves are prevented from returning in a certain cyle, the tree dies and disintegrates.

The parts which sustain their interrelations while other parts are absent from the object provide a context or framework or structure of replacement. Once that framework is gone, however, replacement becomes impossible and the whole ceases, more or less quickly, to exist. The difference between an aggregate XYZ and the type of whole that might constitute a ship or table is that the latter provides, precisely, a framework of replacement, while the mere aggregate provides none at all--which explains the intuitive plausibility of ME as applied to aggregates. The aggregate satisfies ME in virtue of the fact that its wholeness is its elements, and when an element is gone (ceases to exist) so is the whole.

We stand by our point, then, that ME has no intuitive plausibility for wholes in general, and that therefore there is no conflict of intuitions in the sense proposed by Chisholm. On the other hand, Chisholm's effort to find wholes that both satisfy ME and constitute physical objects at a time fails; for while his aggregates satisfy ME, they do not, as such, constitute physical objects, even though it is always possible to consider the parts (pieces) of a physical object at a time simply as an aggregate.

The force of the ship of Theseus puzzle derives from the assumption that there is nothing the same about the ship when all of its parts are replaced by others. But that is because of the prior commitment that all there is to the ship is the pieces, the removeable parts, that enter into it. This is simply false. It is still a ship, and the ship which originated from specific individual parts at a time, which parts were then followed by specific individual others in a specific sequence within a continuing, overall, integrative order. If we make another ship of all the old parts laid aside, as Thomas Hobbes suggested, there is no reason to suppose it might be the ship that resulted from the replacement process in the original ship. It cannot be the same ship as the earlier one, because it came into existence at a different time. It will not be the same even though its originating and present parts, we may assume, are the same as those of the earlier ship at its origination. The individualized structure of replacement is different in the two cases with respect to both the space and the time of its elements or stages.

2. Puzzles of Persistence: Chisholm says (PO 151) that three things count in favor of ME.

(1). ME has a certain intuitive plausibility. I hope that we have dealt with and successfully dismissed this claim, so far as ME is taken as a claim about wholes and parts in general.

(2) ME has the support of an impressive philosophical tradition. This, I think, is less so than Chisholm suggests. In particular his attempt to associate his views with Hume is not very promising. In fact, Hume cannot allow anything like what Chisholm counts as an aggregate; and, on the other hand, Chisholm does not seem prepared, as is Hume, to treat the ens successiva that physical objects are supposed by him to be as fictions. Modes and entia per alio are not for him fictions, as Hume takes enduring physical objects to be, nor are they mental, even "transcendental," constructions, as for Kant. If ME entails or can be supported only on Chisholm's account of substance as aggregate and physical objects as modes that can skip from one substance to another, then philosophical tradition is at least as much against it as for it. Indeed, much more against it.

(3). The fact that ME enables us to deal with what otherwise would be insolvable philosophical puzzles. These have to do, no doubt, with the identity of physical objects such as Theseus' ship, given the replacement of all its parts, as well as with how the same thing can have different properties at different times, e.g., the short boy and the tall man or the piece of metal which at one time is a statue and at another time a vase or just a lump or bar. (RMC 65) Now I do not think that I can here attempt a solution to these puzzles, but I would like to build on what has already been said about the structures or contexts of replacement to suggest a way of understanding the identity of physical objects that persist through time and change of parts.

First a comment about the relevance of the indiscernability of identicals, or the claim that if x and y are identical, then they have all properties in common. Clearly, if this principle is true, and physical objects at different times--with different parts and properties--are to be identical, then parts and properties must be temporally indexed. Here I am going to assume that this can be done,⁸ and accordingly that the indiscernability of identicals by itself offers no obstacle to the persistence of the self-same physical or other temporally enduring object through change. The short boy at T₁ is the tall man of time T₂. One entity has both properties: the property of being-short-at-T₁ and the property of being-tall-at-T₂. This can be extended to the properties of having parts and their replacements.

The indiscernability of identicals aside, however, we still need a better idea or model of the enduring structure or context of replacement that remains throughout the career of the physical object and constitutes its sameness even given some replacement of parts. To assist in this connection we reflect briefly on some aspects of the philosophical tradition, and specifically upon views of Hermann Lotze and Edmund Husserl.

Anti-Atomist Tradition

Chisholm claims to be standing in the atomist and constructivist traditions that is perhaps most clearly associated with Hume, but also, for physical objects at least, with Kant--not to mention more recent thinkers such as Bertrand Russell (in some of his moments), A. J. Ayer and Nelson Goodman, in their different ways. There is much that is right about his claim, with reservations already voiced. But there is also a very strong anti-atomist tradition, certainly including Aristotle and subsequent Aristotelians, the classical Rationalist philosophers, and the Nineteenth and Twentieth Century Idealists, as well as realists such as Hermann Lotze, Edmund Husserl and Alfred North Whitehead. Among contemporary Anglo-American philosophers, P. F. Strawson and Baruch Brody hold for the ontological priority of ordinary physical objects over their basic constituents.⁹ We conclude this paper by explaining some contributions made by Lotze and Husserl to the understanding of the structure of replacement characteristic of physical objects, and how their views might provide conceptual content to the thesis of a restricted mereological essentialism.

The Thing As The Law Of Its States. Lotze's work is especially relevant to understanding the relations of ordinary objects to their parts and properties. For him the first task of metaphysics is to understand what it is about the ordinary object--"The Thing," a phrase of special emphasis for him--that makes it a Thing. (MP 22)¹⁰ Simply put, he finds that to be a Thing is to be a process of change. "It is impossible for that to be unchangeable which we treat as a Thing." (MP 168) The Thing is never static. Its states, with the parts and properties involved in them, are always in transit to some degree, either within itself or in the context of its existence, and so it must be viewed "as a continual becoming." (MP 168, 174) Yet this change is never a mere succession of different entities, but one thing, as common experience finds, with aspects that replace one another within a determinate range and order. "The conception of a Thing which we adopt" involves, he says, "the union of oneness of essential being with multiplicity of so-called states." (MP 165) The difficulty is to express what the 'sameness' is that makes the process one.

Lotze dismisses two familiar accounts of this sameness. One treats it as deriving from a constantly present quality or essence in the Thing's states, and the other treats it as due to the constant presence at all states of a special kind of individual: a unit of "matter," or else a "bare particular" or totally non-qualitative element. In chapters 2 and 3 of Book I of his Metaphysics Lotze goes into great detail to explain why neither of these options can succeed in accounting for the peculiar type of sameness with difference that characterizes the Thing. Positively, he concludes that "the essence is not a dead point behind the activity but is identical with that activity." (MP 75) It is not "a rigid real nucleus" (MP 76) that can be grasped in one perception. (MP 60, 67) Rather, "that which makes a Thing what it is consists only in...a certain regularity with which it changes to and fro within a limited circle of states whether spontaneously or under visible external conditions, without passing out of this circle, and without even having an existence on its own account and apart from any one of the forms which within this circle it can assume." (MP 58) This view is succinctly expressed by equating the Thing--table, ship, tree, etc.--with the law of its states. "The real Thing is nothing but the realized individual law of its procedure." (MP 72) "The Thing is an individual law." (MP 71)

But "how could a law be that which, if simply endowed with reality, would constitute a Thing?" (MP 67) How are we to understand this difficult conception of the thing as law?

First, we must be clear that by "law" is not here meant something abstract or propositional, but the actual ordered sequence in the career of the given Thing. It is the law as real-ized or made concrete and individual. Individualization of the law is a matter of there being actual 'career' stages of physical objects, individuated to space/time coordinates, that are emerging from and passing over into others stages in the career cycle as is appropriate to the kind of object in question. The law itself is not, Lotze holds, a separate structure that forces or causes the states of the physical object to occur as they do (MP 113), and "conformity to law on the part of a Thing would be nothing else than the proper being and behavior of the Thing itself." (MP 71) There is nothing involved in such 'conformity' but the states (with parts and properties) of the Thing passing over in determinate ways into other states of the Thing, up to the point where it ceases to exist.

Now the states do not, for Lotze, merely succeed or accompany one another. There is no "naked coming into being" (MP 78), and no mere existential association. Rather, the states of the object each have inherent tendencies to exist with those that accompany them (and to exclude others) and to give rise to certain others and to them alone. He reclaims (MP 81) the Aristotelian doctrine of potential being as a way of uniting and excluding states that must or cannot enter jointly into the careers that make up physical objects.

In the abstract the career apple tree would be the same for apple tree A and apple tree B. But A and B will be different careers--different "individual laws" or "realized laws"--even though they are of the same abstract type. From the point where they begin to their end, different concrete states enter into them. Once the given tree exists, these states arise out of the previous states in virtue of the determinate potentialities or powers that they contain. The individualized order is what is present, in virtue of the interrelated potentialities and actualities involved at any point, to unify the many states into one entity, this Thing. (MP 149) It is the sameness in and of the Thing.

Lotze invokes the case of music and melody to clarify his view. (MP 69, 73, 146) The melody is present in each of its tones as the ground of it being when and where--and, to a degree, what--it is. This, he thinks, is illustrative of the physical object: of "that permanent yet changeable essence of a Thing." (MP 70)

He did not think, however, that merely by inspecting or philosophically analyzing physical objects themselves we can know that such a unity actually pervades them. (MP 168) Rather, what we know is only that if ships, trees, etc., are Things, they have the kind of unity in difference described. Only an immediate perception, Lotze supposes, could show that this unity actually occurs, and such a perception we have only in the case of our own self and its states. We experience the relation of our states to the identity that runs through them--somewhat as the one melody is present in each of its tones--and makes them the states of the same thing. "It is only through the fact that our attention, bringing events into relation, comprehends past and present in memory, while at the same time there arises the idea of the persistent Ego to which both past and present belong, that we become aware what is meant by Unity of Being throughout a change of manifold states, and that such unity is possible." (MP 169)

If we assume such careers for physical objects as is suggested by Lotze's analysis, then the limits imposed by the structure of replacement relevant to such objects and their pieces, with its inherent possibilities and limitations, may be understood as follows: Pieces whose replacement, in an appropriate manner, does not destroy the cycle, are not essential to their wholes. If replacement does destroy the cycle, they are essential to their wholes.

The line between essential and non-essential parts of a whole may be hard to draw in many cases. But there also are clear cases of essential and of non-essential parts. A leg can be replaced on an ordinary table, a tire on a car. It is the understanding of what the 'career' of the object is that guides us to the objective context or structure of replacement, where we may interact to sustain the ongoing life of a whole, or else write it off as finished. The entire table top (not just its surface or a part of it) cannot be removed and the same table restored from the legs only. A melody might omit a note, or flat or sharp several, while remaining the same melody. But "Variations on a Theme by Paganini" is not the theme by Paganini, nor conversely.

A Structure Resting On Ideal Laws. The views of Lotze are helpful in bringing some clarification to what the structure of replacement that determines the essential and unessential pieces of the physical object might be. The basic idea of the Thing as having a determinate career within the confines of law is adopted by Husserl. But the interweaving potencies that generate the unity of the physical object for Lotze are in effect taken by him as ultimates, not to be further conceptualized. This seems to me a mistake, in view of the fact that these potencies are obviously a function of the universals or properties that qualify the states of the object or even the object as a whole. Lotze rightly sees that "abstract" entities in themselves do not give rise to change. But he does not do justice to the fact that, when exemplified, they achieve a certain dynamism: they account for the determinate character of the potentialities of interrelation that the stages and parts of physical objects have. We turn to Husserl for a more precise account of the ontological structure of the physical object and its various kinds of parts.

Husserl begins his official treatment of part and whole from a discussion of "sense contents" or sense data; but what interests him is not, in the first instance, their pieces or fragments, as we saw in G. E. Moore and Chisholm. Rather, he is interested in the existential dependence between aspects or "moments" of sense contents, such as extension (or spatiality) and color in a visual sense content, or the pitch and intensity of an auditory sense content (a tone). (LI 440f)¹¹ Husserl observes that these "moments" often exhibit a characteristic type of inseparability, such that, for example, if extension is taken away the color does not remain, and conversely. Such 'parts' cannot exist outside of the whole that they constitute, nor does the whole survive their disunion. Other sense contents, such as those famously introduced by Berkeley corresponding to the head of a horse (LI 438f), are separably presentable and capable of existing continuously through a complete variation of the sense contents surrounding them. (LI 443) This is true with respect to "every phenomenal thing and piece of a thing." (LI 439)

But Husserl does not find here a mere brute fact of dependence on independence of sense contents. Rather, the existential dependence (or lack thereof) is derivative from the essence or nature of the contents concerned, from the properties constitutive of what they are. Pitch, in a tone, has a certain character that requires intensity also to be present in its instances. The same is true for color and extension. But not for the horse-head image and its surroundings in the sense field. And this is the crucial step beyond Lotze that we find in Husserl's account. It is no longer 'blind' potentialities that move and structure the process of change, but rather the connections (Ideal Laws) and lack thereof between the properties inherent in entities and their states. Pitch is not just factually dependent upon intensity--as if the next pitch we find might very well be without any degree of loudness. It is essentially so, and, Husserl is sure, can be seen to be so. Similarly for the cases of independence: "A thing or a piece of a thing can be presented by itself, meaning that it would be what it is even if everything outside it were annhilated." (LI 445) This independence also is a matter of the essence and Ideal Laws involved in the case, and remains so even though, factually, things and their pieces, as well as the corresponding sense contents, exist only under certain conditions imposed by nature, and hence in some sense are dependent.

Although such distinctions were originally drawn by Husserl (following Carl Stumpf) in a study of sense contents, he points out that the difference between the dependent (or "abstract") and the independent (or "concrete") involves no essential connection with the mental. (LI 444) It is a completely general ontological distinction, applying to all types of entities: the Ideal or universal (LI 448) and the real ("Things"), as well as the psychical. The dependencies in question are "objectively-ideal necessities of an inability to be otherwise....given in our consciousness of apodictic self-evidence." (LI 446) The dependent object has an "essence, its pure species, which predestines it to partial being.... In the case of independent objects such a law is lacking: they may, but need not, enter into more comprehensive wholes." (LI 44f; cf. 454f)

Since the necessities and possibilities of dependence and independence that show up between parts and parts, and wholes and parts, are conditioned upon the specific qualities of those wholes and parts, they are not analytic or formal necessities and possibilities, but synthetic ones. Thus, "all the laws or necessities governing different sorts of non-independent items fall into the spheres of the synthetic a priori." (LI 456ff)

This synthetic a priori character carries over to the general laws of foundation, and of wholes and parts, which Husserl formulates in Chapter 2 of the IIIrd "Logical Investigation." If a law or connection of essence dictates that A, whatever entity it may be, cannot exist except in a whole unifying it with another thing M, then, Husserl says, "an A as such requires foundation by an M," or "an A as such needs to be supplemented by an M." (LI 463) Moreover, if the existence of A requires M, then "a whole including an A but no M cannot satisfy A's need for supplementation and must share it." (LI 464) In other words, whatever founds a necessary part of a whole founds the whole.

From these beginnings Husserl elaborates a subtle and intrinsically interesting theory of whole and part. In general, this theory, correctly I believe, takes the relation of whole and part to be definable in terms of his "foundation" relation (see LI 475), and foundation to be a matter of the Ideal Laws of--the inherent connections and disconnections between--properties or essences that constitute the natures of the wholes and parts in question. "The only true unifying factors, we may firmly state, are relations of 'foundation'. The unity even of independent objects is in consequence brought about by 'foundation'. Since they are not, as independent objects, 'founded' on one another, it remains their lot to 'found' new contents themselves,..." (LI 478) And this last point applies specifically to physical objects and their pieces.

For our purposes here, on Husserl's theory the pieces of physical objects, such as Chisholm has considered, have the interplay they do in their wholes only through the moments that make them up and through the laws that govern those moments and, thereby, the pieces in relation to their relevant wholes---ships, tables and trees. (See LI 482-488) This then opens the way for "modified Ideas of empirical 'foundation', of empirical wholes, and empirical independence and non-independence." (LI 486), which go beyond strict laws of essence. "Nature with all its physical laws is a fact that could well have been otherwise." (Ibid.) The case of the dependence of the tree upon its leaves, discussed above, is not a matter of essence, but of natural law. A theory of whole and part in terms of the 'foundation' relation will therefore require an extension if it is to be adequate to physical objects as we know them. Husserl achieves this extension by allowing us to "treat natural laws, without regard to their infection with contingency, as true laws, and to apply to them all the pure concepts we have formed." (LI 486)

* * * * *

Now we cannot here defend the views of Lotze and Husserl relevant to physical objects and their parts, nor do we suggest that they are on the whole defensible. Indeed, they obviously pose serious philosophical problems in their own right--though I do regard Husserl's account as one of the better possibilities. Our aim has been to present them as serious possible accounts of what the structure of replacement, permitting a restricted version of mereological essentialism, might look like. At least we can cite them as illustrations of how such a structure might be interpreted.

But we can also say with assurance that mereological essentialism depends, even for its intelligibility as a claim, upon making sense of the idea of replacing parts. ME may be applied to aggregates as a limiting case. An aggregate is essentially the type of 'entity' which does not admit of replacement of parts. But this is simply because it provides no structure of replacement at all. 'Replacement' of a member of an aggregate cannot leave the same group of interrelated things because the aggregate is straightforwardly identical with its elements. Such wholes as physical objects sharply contrast with this. When some of their parts, though certainly not all, are removed, a structure of replacement remains--at least for a time--that may permit replacement of parts and the restoration of the whole. It is this enduring or continuous structure of replacement that constitutes the identity of the physical object through the replacement--and sometimes even the enduring lack--of some of its parts. Lotze and Husserl helpfully comment on how this structure might be understood.

NOTES