没想到逻辑哲学课程分配给我的读书任务是唯一找不到中文译文的…………甚至连英文电子版都很难找。
终于找到了pdf版的原文，http://www.jstor.org/view/00294624/di982797/98p0002q/0
自己用汉王文本王OCR识别了一下，初步校改后如下。搞成电子版后借助金山词霸读起来稍微方便一点点吧……不过还是好痛苦……
Identity through Possible Worlds: Some Questions
Roderick M. Chisholm
Noûs, Vol. 1, No. 1. (Mar., 1967), pp. 1-8.
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Identity Through Possible Worlds:
Some Questions
RODERICK M. CHISHOLM
BROWN UNIVERSITY
It is now easy to see a simple way of avoiding undesirable existential generalizations in epistemic contexts. Existential generalization with respect to a term–say b–is admissible in such contexts if b refers to one and the same man in all the “possible worlds” we have to consider.
—Hintikka[1]
1 In an article on Hintikka’s Knowledge and Belief, I suggested that certain difficult questions come to mind when we consider the thought that an individual in one possible world might be identical with an individual in another possible world.[2] The present paper is written in response to the editor’s invitation to be more explicit about these questions.
2 Let us suppose, then, that the figure of an infinity of possible worlds makes good sense and let us also suppose, for simplicity of presentation, that we have a complete description of this one. We may consider some one of the entities of this world, alter its description slightly, adjust the descriptions of the other entities in the world to fit this alteration, and then ask ourselves whether the entity in the possible world that we thus arrive at is identical with the entity we started with in this world. We start with Adam, say; we alter his description slightly and allow him to live for 931 years instead of for only 930; we then accommodate our descriptions of the other entities of the world to fit this possibility (Eve, for example, will now have the property of being married to a man who lives for 931 years instead of that of being married to a man who lives for only 930); and we thus arrive at a description of another possible world.[3]
3 Let us call our present world “W1” and the possible world we have just indicated “W2”. Is the Adam of our world W1 the same person as the Adam of the possible world W2? In other words, is Adam such that he lives for just 930 years in W1 and for 931 in W2? And how are we to decide?
4 One’s first thought might be that the proposition that Adam is in both worlds is incompatible with the principle of the indiscernibility of identicals. How could our Adam be identical with that one if ours lives for just 930 years and that one for 931? Possibly this question could be answered in the following way:
5 “Compare the question: How can Adam at the age of 930 be the same person as the man who ate the forbidden fruit, if the former is old and the latter is young? Here the proper reply would be: it is not true that the old Adam has properties that render him discernible from the young Adam; the truth is, rather, that Adam has the property of being young when he eats the forbidden fruit and the property of being old in the year 930, and that these properties, though different, are not incompatible. And so, too, for the different possible worlds: It is not true that the Adam of W1 has properties that render him discernible from the Adam of W2; the truth is, rather, that Adam has the property of living for 930 years in W1 and the property of living for 931 in W2, and that these properties, though different, are not incompatible.”
6 I think it is clear that we must deal with the old Adam and the young Adam in the manner indicated; but in this case, one could argue, we know independently that the same Adam is involved throughout. But are we justified in dealing in a similar way with the Adam of W1 and the Adam of W2? In this latter case, one might say, we do not know independently that the same Adam is involved throughout. Here, then, is one of the questions that I do not know how to answer. Let us suppose, however, that we answer it affirmatively.
7 The Adam of this world, we are assuming, is identical with the Adam of that one. In other words, Adam is such that he lives for only 980 years in W1 and for 981 in W2. Let us now suppose further that we have arrived at our conception of W2,
not only by introducing alterations in our description of the Adam of W1, but also by introducing alterations in our description of the Noah of W1. We say; “Suppose Adam had lived for 981 years instead of 980 and suppose Noah had lived for 949 years instead of 950.” We then arrive at our description of W2 by accommodating our descriptions of the other entities of W1 in such a way that these entities will be capable of inhabiting the same possible world as the revised Noah and the revised Adam. Both Noah and Adam, then, may be found in W2 as well as in W1.
8 Now let us move from W2 to still another possible world W3. Once again, we will start by introducing alterations in Adam and Noah and then accommodate the rest of the world to, what we have done. In W3 Adam lives for 982 years and Noah for 948. Then moving from one possible world to another, but keeping our fingers, so to speak on the same two entities, we arrive at a world in which Noah lives for 980 years and Adam for 950. In that world, therefore, Noah has the age that Adam has in this one, and Adam has the age that Noah has in this one; the Adam and Noah that we started with might thus be said to have exchanged their ages. Now let us continue on to still other possible worlds and allow them to exchange still other properties. We will imagine a possible world in which they have exchanged the first letters of their names, then one in which they have exchanged the second, then one in which they have exchanged the fourth, with the result that Adam in this new possible world will be called “Noah” and Noah “Adam.” Proceeding in this way, we arrive finally at a possible world Wn which would seem to be exactly like our present world W1, except for the fact that the Adam of Wn may be traced back to the Noah of W1 and the Noah of Wn may be traced back to the Adam of W1.
9 Should we say of the Adam of Wn that he is identical with the Noah of W1 and should we say of the Noah of Wn that he is identical the Adam of W1? In other words, is there an x such that x is Adam in W1 and x is Noah in Wn, and is there a y such that y is Noah in W1 and y is Adam in Wn? And how are we to decide?
10 But let us suppose that somehow we have arrived at an affirmative answer. Now we must ask ourselves: How is one to tell the difference between the two worlds W1 and Wn? Shall we say that, though they are diverse, they are yet indiscernible from each other–or, at any rate, that the Adam of W1 is indiscernible from the Adam of Wn (who is in fact the Noah of W1) and that the Noah of W1 is indiscernible from the Noah of Wn (who is in fact the Adam of W1 )? There is a certain ambiguity in “discernible” and in “indiscernible”. The two Adams could be called “discernible” in that the one has the property of being Noah in the other world and the other does not, and similarly for the two Noahs. But in the sense of “indiscernible” that allows us to say that “Indiscernibles are identical” tells us more than merely “Identicals
are identical,” aren’t the two Adams, the two Noahs, and the two worlds indiscernible? Could God possibly have had a sufficient reason for creating W1 instead of Wn?
11 If W1 and Wn are two different possible worlds, then, of course, there are indefinitely many others, equally difficult to distinguish from each other and from W1 and Wn. For what we have done to Adam and Noah, we can do to any other pair of entities. Therefore among the possible worlds which would seem to be indiscernible from this one, there are those in which you play the role that I play in this one and in which I play the role that you play in this one.[4] (If this is true, there may be good ground for the existentialist’s Angst; since, it would seem, God could have had no sufficient reason for choosing the world in which you play your present role instead of one in which you play mine.)
12 Is there really a good reason for saying that this Adam and Noah are identical, respectively, with that Noah and Adam? We opened the door to this conclusion by assuming that Adam could be found in more than one possible world—by assuming that there is an x such that x is Adam in W1 and lives here for 930 years and x is also Adam in W2 and lives there for 931. If it is reasonable to assume that Adam retains his identity through the relatively slight changes involved in the transition from W1 to W2, and so, too, for Noah, then it would also seem reasonable to assume that each retains his identity through the equally slight changes involved in all the other transitions that took us finally to Wn. (These transitions, of course, may be as gradual as one pleases. Instead of it being a year that we take away from Noah in our first step and give to Adam, it could be only a day, or a fraction of a second.)But identity is transitive. And therefore, one might argue, once we allow Adam to exist in more than one possible world, we commit ourselves to affirmative answers to the puzzling questions we have encountered.
13 Is there a way, then, in which we might reasonably countenance identity through possible worlds and yet avoid such extreme conclusions? The only way, so far as I can see, is to appeal to some version of the doctrine that individual things have essential properties. One possibility would be this:
14 For every entity x, there are certain properties N and certain properties E such that: x has N in some possible worlds and x has non-N in others; but x has E in every possible world in which x exists; and, moreover, for every y, if y has E in any possible world, then y is identical with x. (If “being identical with x” refers to a property of x, then we should add that E includes certain properties other than that of being identical with x.) The properties E will thus be essential to x and the properties N non-essential, or accidental.[5]
15 To avoid misunderstanding, we should contrast this present use of “essential property” with two others.
16 (1) Sometimes the “essential properties” of a thing are said to be just those properties that the thing has necessarily. But it is not implausible to say that there are certain properties which are such that everything has those properties necessarily; the properties, for example, of being either red or non-red, of being colored if red, and of being self-identical.[6] Thus the Eiffel Tower is necessarily red or non-red, necessarily colored if red, and necessarily self-identical; and so is everything else.[7]
17 (2) And sometimes it is said (most unfortunately, it seems to me) that each individual thing is such that it has certain properties which are essential or necessary to it “under certain descriptions of it” and which are not essential or necessary to it “under certain other descriptions of it.” Thus “under one of his descriptions,” the property of being President is said to be essential to Mr. Johnson whereas “under that description” the property of being the husband of Lady Bird is not; and “under another one of his descriptions,” it is the other way around. Presumably every property P of every individual thing x is such that, “under some description of x,” P is essential or necessary to x.
18 But if E is the set of properties that are essential to a given thing x, in the sense of “essential” that we have defined above, then: E will not be a universal property (indeed, nothing but x will have E): some of the properties of x will not be included in E; and E will not be such that there are descriptions of x “under which” E is not, in the sense defined, essential to x.
19 If we accept this doctrine of essential properties, we may say, perhaps, that the property of living for just 930 years is not essential to Adam and therefore that he may inhabit other possible worlds without living for just 930 years in each of them. And so, too, perhaps, for having a name which, in English, ends with the letter “m”. But, we may then go on to say, somewhere in the journey from W1 to Wn, we left the essential properties of Adam (and therefore Adam himself) behind. But where? What are the properties that are essential to Adam? Being the first man? Having a name which, in English, begins with the first letter of the alphabet? But why these properties? If we can contemplate Adam with slightly different properties in another possible world, why can’t we think of him as having ancestors in some possible worlds and as having a different name in others? And similarly for any other property that might be proposed as being thus essential to Adam.
20 It seems to me that even if Adam does have such essential properties, there is no procedure at all for finding out what they are. And it also seems to me that there is no way of finding out whether he does have any essential properties. Is there really a good reason, then, for supposing that he does?
21 The distinction between essential and non-essential properties seems to be involved in one of the traditional ways of dealing with the problem of knowing who.[8] If this way of dealing with that problem were satisfactory, then the doctrine of essential properties might have a kind of independent confirmation. But I am not sure that is satisfactory. The problem of knowing who may be illustrated in this way. I do not know who it was who robbed the bank this morning, but I do know, let us assume, that there is someone who robbed the bank and I also know that that person is the man who drove off from the bank at 9:20 A.M. in a Buick Sedan. For me to know who he is, therefore, it is not enough for me to have information enabling me to characterize him uniquely. What kind of information, then, would entitle me to say that I know who he is? The essentialistic answer would be: “You know who the bank robber is, provided that there is a certain set of properties E which are essential to the x such that x robbed the bank and you know that x has E and x robbed the bank.” But if my doubts about essential properties are well-founded, this solution to the problem of knowing who would imply that the police, though they may finally “learn the thief’s identity” will never know that they do. For to know that one knows who the thief is (according to the proposed solution) one must know what properties are essential to the thief; and if what I have said is correct, we have no way of finding out what they are. How are the police to decide that they know who the thief is if they have no answer to the metaphysical question “What are the essential properties of the man we have arrested?”[9]
22 It is assumed, in many writings on modal logic, that “Necessarily, for every x, x is identical with x” implies “For every x, necessarily x is identical with x,” and therefore also “For every x and y, if x is identical with y, then necessarily x is identical with y.” But is the assumption reasonable? It leads us to perplexing conclusions: for example, to the conclusion that every entity exists in every possible world and therefore, presumably, that everything is an ens necessarium.
23 Why assume that necessarily the evening star is identical with the evening star? We should remind ourselves that “The evening star is identical with the evening star” is not a logical truth, for it implies the contingent proposition “There is an evening star,” and that its negation is not “The evening star is diverse from the evening star.” Wouldn’t it be simpler to deny that “Necessarily, for every x, x is identical with x” implies “For every x, necessarily x is identical with x”? Then we could deny the principle de dicto, “Necessarily the evening star is identical with the evening star,” and also deny the principle, de re, “The evening star is necessarily identical with the evening star.”[10] We could still do justice to the necessity that is here involved, it seems to me, provided we continued to affirm such principles, de dicto, as “Necessarily, for every x, x is identical with x” and “Necessarily, for every x and y, if x is identical with y then y is identical with x,” and such principles, de re, “The evening star, like everything else, is necessarily self-identical.”
[1] Jaakko Hintikka, Knowledge and Belief: An Introduction to the Logic of the Two Notions (Ithaca: Cornell University Press, 1962), p. 152.
[2] “The Logic of Knowing,” 1ournal of Philosophy, LX (1968), pp. 778-795; see especially pp. 787-795.
[3] It should be noted that the possible world in question is not one that Hintikka would call epistemically possible, for it could be said to contain certain states of affairs (Adam living for 931 years) which are incompatible with what we know to hold of this world; hence it is not one of the worlds Hintikka is concerned with in the passage quoted above. But it is logically possible, and that is all that matters for purposes of the present discussion.
[4] “She (Ivich) looked at the glass, and Mathieu looked at her. A violent and undefined desire had taken possession of him; a desire to be for one instant that consciousness . . . to feel those long slender arms from within . . .To be Ivich and not to ,cease to be himself.” Sartre, The Age of Reason. Compare N. L. Wilson, “Substance without Substrata,” Review of Metaphysics, XII (1959), and A. N. Prior, “Identifiable Individuals,” Review of Metaphysics, XIII (1960).
[5] We could put the doctrine more cautiously by saying that the distinction between the two types of property holds, not for every entity x, but only for some entities x. But what reason could there be for thinking that it holds of some entities and not of others?
[6] Sometimes these properties are called “analytic properties” or “tautological properties”; but the property of being colored ff red should not be socalled if, as some have argued, “Everything that is red is colored” is not analytic.
[7] From the proposition that the Eiffel Tower is red and necessarily colored if red, it would be fallacious to infer that the Eiffel Tower is necessarily colored; this is the fallacy of inferring necessitate consequentiae from necessitate consequentiae. And from the proposition that the Eiffel Tower is necessarily red or non-red, it would be fallacious to infer that the proposition that the Eiffel Tower is red or non-red is a necessary proposition; the proposition could hardly be necessary, for it implies the contingent proposition that there is an Eiffel Tower. This latter fallacy might be called the fallacy of inferring necessitate de dicto from necessitate de re.
[8] Compare Aristotle, De Sophisticis Elenchis, 179 b 3; Petrus Hispanus, Summulae Logicales, ed. I. M. Boehenski (Turin, 1947), 7.41; Franz Brentano, Kategorienlehre (Leipzig, 1933), p. 165.
[9] Hintikka says that we know who the thief is provided that there exists an x such that we know that the thief is identical with x (op. cir., p. 153). But under what conditions may it be said that there exists an x such that we know that the thief is identical with x? Presumably, if ever, when we catch him in the act-when we see him steal the money. But the teller saw him steal the money and she doesn’t know who he is. I have suggested elsewhere a slightly different way of looking at these questions; compare op. cir., pp. 789-791, and “Belietng and Intentionality,” Philosophy and Phenomenological Research, XXV (1964), pp. 266-269, esp. p. 268.
[10] I have discussed this possibility in “Query on Substitutivity,” in Boston Studies in the Philosophy of Science, Vol. II, ed., Robert S. Cohen and Marx W. Wartofsky (New York: The Humanities Press, 1965), pp. 275-278.
If we deny that “Necessarily, for every x, x is F” implies “For every x, necessarily x is F,” then presumably we should also deny that “It is possible that there exists an x such that x is F” implies “There exists an x such that it is possible that x is F.” But isn’t this what we should do? One could hold quite consistently, it seems to me, that though it is possible that there exists something having the properties that Christians attribute to God, yet nothing that does exist is su.ch that it is possible that that thing has the properties that Christians attribute to God.
逻辑哲学课读Identity through Possible Worlds: Some Questions的报告
星定 发表于 2006-06-08 11:01:54
1
——问题：一个可能世界的某个个体如何可能与另一个可能世界中的某个个体“同一”。
2
——可能世界理论将意味着什么呢？（齐硕姆是可能世界理论的反对者）我们对这个世界的某个事物的描述进行微调，然后调整其它事物的描述以适合此改造，然后说在那个改造后的可能世界中的那个事物与在这个世界中的这个事物同一。比如以亚当为例：我们略微改动一下他的寿命，从930年改变到931年，于是对于其它一些事物的描述也要做出相应的调整，例如夏娃原来是嫁给了一个活了930年的男人，而现在她嫁给了一个活了931年的男人。完成了这些调整后，我们便到达了另一个可能世界。
3
——简称当前的世界为W1，所及的那个可能世界为W2，我们如何判断说亚当就是在W1中活了930岁以及在W2中活了931岁的那个人？
4
——说亚当同时在两个世界中显然是违反了莱布尼茨的“同一不可分辨原则”——也就是说两事物同一当且仅当两事物的所有属性全同。
5
——对此问题可能的回答是：首先比较这个问题：930岁时的老亚当如何与偷食禁果时的亚当是同一个人呢？合理的回答将是：亚当在偷食禁果时具有“年轻”这一性质，而930岁的亚当具有“老”这一性质，这两个性质固然是不同的，但并不冲突。同样地，在不同的可能世界中，930岁与931岁固然不同，也并没有导致两个亚当不可辨认为同一个人。
6
——但是诚然可以这样来处理老亚当和年轻亚当的问题，但是把这种思路应用在跨界统一性问题上是合法的吗？在前一问题中，我们可以独立地知道那贯穿始终的是同一个亚当（大概是指其变化是连续的），但在跨世界问题上，如果我们也承认略有不同的亚当为同一的，那么……
7~11
——如果我们假定W1上活930年的亚当与W2上活931年的亚当是同一个，那就会出现问题，比如可以不断地微调从而把亚当变变变一直变成诺亚，同时把诺亚搞成亚当，那就乱套了云云。/——我认为这些论述很没有意思，我们很容易就能想到他是怎么“变”，但他玩这套戏法不能说明什么问题，与其说是说明了跨界同一性的荒谬性，还不如说这意味着跨界同一性的传递性是不合理的。总的来说，齐硕姆始终以莱布尼茨对于同一性的标准来作为衡量的标准，出现的荒谬结论正是说明莱布尼茨的同一性定义是不合理的。
12
——齐硕姆分析说，造成这样荒谬结论的前提假设是亚当可以在不同的可能世界中找到，一旦假定这一点，就无可避免地导致荒谬的结果：“once we allow Adam to exist in more than one possible world, we commit ourselves to affirmative answers to the puzzling questions we have encountered.”/——但是，这个结论显然不是仅仅由那单独一条前提就推出来的，如果出现了荒谬的结论，需要检查整个推理过程以及每一条前提假设，而齐硕姆对于在推导中用到的其它前提要么是忽视了，要么是轻描淡写地说句“显然”了事，如“But identity is transitive”同一性是（过渡的？传递的？）——为什么对同样作为前提的这条论断如此轻易地放过了呢？除了跨界同一性之外，值得怀疑的前提假设还有：同一性是否是传递的？是否所有的性质都可以通过微调转变——即便说几粒谷子和一堆谷子之间可以连续地变化好了，那么有谷子与没谷子也可以连续转化吗？即便说930岁可以变到931岁，但可以把亚当变成女的吗？正如陈老师也提到，本质主义也并非一无是处的。齐硕姆在后文提到了本质主义，不过对同一性的传递性从未进行反思，我在文后将讨论这一问题。
13
——有没有什么途径可能在避免这些极端荒唐的结论的同时支持跨界同一性呢？我目前能想到的唯一途径是（本质主义）。
14
——本质主义的说法是对事物x，区分其两种不同的属性，一种是本质的、主要的——E，一种是非本质的、次要的——N，在不同的世界里包含或不包含N的事物，只要都包含E，就是同一的。
15
——为了避免误会区分一下两种：
16
——一种本质（必然性质）是例如“红或非红”、“可能红”、“自我同一”等，这些确实是不变的，但这些是任何事物都拥有的性质。
17
——另一种就是说，在对一个事物的某一个描述下有某些属性是本质的或必要的，而在其它描述中这些属性则不是本质的或必要的。也就是说，在一种描述下，“是美国总统”对于约翰逊而言是必要的属性，而“是伯德的丈夫”则不是本质属性；而在另一种对他的描述下，正好相反。大概对每个事物x，的每一个属性P都是类似——“在对x的某些描述下”P对于x是本质的或必然的。/——我认为，这恰恰是说明了“在某些描述下”对于同一性讨论的重要性。）
18
——但是这样的“本质属性”P不满足之前对于同一性的定义，即“x且只有x才有P”。/——（但是为什么始终不敢质疑之前的定义呢？）
19
——如果我们接受了这种本质主义，我们可能说：或许，活了930年不是亚当的本质属性，所以同一个亚当可能在不同的世界中拥有不同的寿命等等。但是，在从W1到Wn的过程中，我们却把亚当的“本质”遗失了，但是在什么地方丢了呢？究竟什么才是亚当的本质？第一个男人？名字中的最后一个字母？为什么？
20
——在我看来似乎即便说亚当有某些本质的属性，也没有任何能够找出它们的手段。这也就意味着没有任何途径找出他到底有没有某些本质的属性。
21
——本质与非本质的分辨看起来与一个传统问题的讨论有关。如果对于那个问题的处理是令人满意的，本质主义可能被稍稍有所支持。但我不确信那是令人满意的。——我不知道谁在早上抢了银行，但我知道有一个抢银行的人并且那个人在早上9:20 乘Buick Sedan离开银行。…………但警察如何能够在不能回答被捕者的本质是什么这一问题的情况下确定他是那个小偷呢？
22~23
——关于从物逻辑与从言逻辑，参考《逻辑哲学》P327页以下
我的观点：
首先，齐硕姆始终坚持对莱布尼茨的同一性定义毫不怀疑，这使得后面的讨论越走越远。
我认为莱布尼茨对同一性的定义是不合理的，它顶多是数学上的同一性，例如说等边三角形的重心与垂心“同一”，但用那种定义顶多只能分析一个静态世界中的同一，却难以分析任何一种现实世界中的关系。例如齐硕姆已经提到的，我们凭什么说年轻的亚当与年老的亚当是同一个人？他在文中的处理十分轻描淡写，似乎是说存在一个同一的亚当贯穿始终，但我们问的就是如何能说有一个同一的亚当贯穿始终？无论如何，按照莱布尼茨的定义，我不能说现在的我和今天早上的我是“同一”的，那么现在的我与今天早上的我究竟是何关系？显然，早上的我与现在的我是同一个人，但按照莱布尼茨却不能说两者是同一的，那么究竟是谁搞错了？更换了一个零件的轮船与原来的轮船之间是什么关系？如果说是同一艘船，那么我们也能通过累积的微调把一艘轮船换成另一艘，再把换下来的全部零件再组成一艘和原来一模一样的轮船，在两艘轮船间换来换去这一古老的问题这就好像齐硕姆在亚当和诺亚之间变来变去类似，可以说，齐硕姆并没有提出什么跨界同一性的新问题，只是把古老的同一性问题放在模态逻辑的语境下重新提起罢了。
其二，为什么同一性必须是传递的？W1同一于W2、W2同一于W3，就一定能说W1同一于W3了吗？无论这一前提是否合理，齐硕姆将这一前提不加怀疑地轻易放过，作为逻辑学家而言无疑是失职的。
在模态逻辑中，一个可能世界语义学的“模型”是〈W，R，V〉，W是所有可能世界的集合，V是赋值，而关键不可忽略的是R——即可能世界中的通达关系。在齐硕姆的整个探讨中，是不考虑R的，或者把R设定为全通关系。但事实上，并非任何模态逻辑系统都要求可能世界间的关系必然是传递的。
还有一个容易被忽略的问题——为什么承认跨界同一性的人，都一定承认930岁的亚当与931岁的亚当是同一的？是否可以在承认有跨界同一性的同时，认为不能说微调后的个体之间同一呢？我认为，不允许无条件的微调的同时也可以支持跨界同一性。这就是我下面要说的——语境对于跨界识别的重要性。例如：在我们谈论“如果亚当的寿命931年，那么……”这一问题时所涉及的可能世界中，那个活了931年的亚当将被识别为与初始世界中的亚当“同一”，但在我们讨论“如果亚当的英语拼写的最后一位是n，那么……”这一问题时所涉及的可能世界中，亚当将不允许是活了931年的。在下面具体解释：
我认为，谈论A与B“同一”，恰恰与”under which description”是密切相关的，换句话说，与我们希望谈论两者有何“不同点”是有关的。我们谈论两者有何差异的方式也是我们判断两者之“同一”的要素之一。这并非胡说八道，例如说我们如何可能谈论“变化”？在何种情况下才有资格谈论“A变成了B”？事实上，如果我们不承认A、B的共同点，不承认A、B间有某种“不变”的东西，也就是说A、B是两个完全不同的东西的话，又怎么能谈论变化呢。这里，我要说：变与不变是交织在一起的，脱离了一方则无法谈论另一方。
这是不是玩弄辩证法玩戏法呢？是否只能在同与不同之间循环定义呢，并不是那样，因为谈论“变化”确实需要预设“不变”，但我们有权直接谈论“不同”，“A具有性质p、B没有性质p”——这在逻辑上是很清楚的。我试图通过这里来解释何谓同一性。
首先，我们凭什么说现在的我与今天早上的我是同一个人呢？关于同一性问题，我认为蒯因在《从逻辑的观点看》一书中讲得很好，他提到了“不能两次踏入同一条河流”这一说法，蒯因指出：“‘这条河流’的意思是‘包含这一瞬间对象在内的一些瞬时对象的、具有河流特点的总合’”（第62页）蒯因指出“在实指时，空间的范围与时间的范围不是可以完全分开的”。我认为蒯因的洞见是深刻的，从莱布尼茨到齐硕姆，他们脑子里对于实指的理解仅仅局限于静态的、理想的、数学的世界，而现实的问题靠那种世界观是没有办法恰当地分析的。即便他们的逻辑严密，但一点现实问题分析不了又有何用？
我认为，“我”并非时间过程中的一个点，“我”这个东西，包括的是由我的经历所组成的整个的“过程”（蒯因也提到“过程”一词。之所以说现在的我与早晨的我是同一的，是因为直到早晨之前，这两个东西的经历、过程都是同一的。这两个我的区别在于其中一个多经历了半天，另一个没有，把与我们所谈论的这一区分有关的所用属性去掉，两者的剩下的属性是全同的，因此说这两者是同一的。
推广来看，在跨界识别上：例如，我们说在一个可能世界中我今天吃了早饭。而在另一个可能世界中我没吃，如何在这两个世界中分辨出同一个我呢？方法是：首先考察我们所谈论的话题——有没有吃早饭，然后把两个世界的那两个人身上依赖于这一事件的属性去掉，比如我今天中午的状态取决于我是否吃了早饭，所以暂时把这些属性去掉，剩下的是那两个人的与是否吃了早饭无关的所有属性，例如此处就是在今天早上以前的全部经历、状态和过程，如果这些是全同的，那么我们便说这两个人“同一”。需要说明的是：事实上，“可能世界”正是我们按照论题的需要设计出来的，所以实际的操作并不是在两个世界里做分辨，而是说我们在设计可能世界时需要遵循的规则是：被称作同一的两者，除了我们所谈论的属性以及依赖于此的属性之外，其它属性全部一致。例如我们在讨论“如果亚当多活一年，那么……”时，设立了一个亚当活了931年的可能世界，但是在那个世界上我们认为亚当仍然拼作“Adam”，如果我们还要继续讨论下一个问题“如果亚当的末字母是n”，那就是再设立了新的可能世界的“分支”。这第二级的分支的世界是原世界的第一级的可及世界的可及世界，但是可及世界的可及世界是否仍是可及世界则需要视具体情况而定。
我的意思是：倘若分别在两个可能世界上的A与B两者在性质p上是不同的，而当我们所谈论的话题是“如果p/非p，则……”——例如谈论“如果我没有吃早饭，那么……“时，在我决定吃早饭之前我的所有属性都与此话题无关，让我们来考察A与B所拥有的所有与p无关的性质。如果A与B所拥有的所有与p无关的属性完全一致，那么便称可判断A、B是同一个谈论对象。也就是说，比如在两个可能世界中各有一个人，其中一个吃了早饭，另一个没有吃，那么比如没吃早饭的那个人在中午感觉很饿，但“在中午感觉饿”这一属性与是否吃了早饭是有关的。但这个人昨天做的事则与之无关。如果说两个人除了与是否吃过早饭相关的属性之外，在一切方面保持一致，便说我们是谈论同一个对象。反过来说，如果我们希望在两个世界间谈论同一个事物，就要求我们认为这两个个体除非是在特别指定下的区别以及依赖于那些区别的属性，其它一切方面尽量多地保持一致。
当然，之前的分析是过于简化了的，在实际运用可能世界语义学时，“不同”的设定往往并不会这样明确。例如我们所“必然”指该命题“在所有的可及的可能世界中都为真”，那么说“x必然有性质p”时，如何在所有的可及的可能世界中分辨出个体x的所在呢？我想，只是抽象地谈论确实是说不清楚的，但无论如何，我认为要考虑跨界同一性问题，必不能忽略可能世界模型中的“R”——我们如何针对谈论某个问题的需要而设计可能世界模型？如何设计可及关系？这就是谈论跨界同一性的语境，我们如何分辨W1上与W2上的跨界同一性有赖于之前我们是如何区分了W1与W2，又是如何把它们联系起来，
2006年6月7日
