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Identity And Universals: A Conceptualist Approach to Logical, Metaphysical, and Epistemological Problems of Contemporary Identity Theory
Chapter 2, The Things That Are Not: Against Science Fiction in Identity Theory
by Carolyn Ray

Date: 11 Nov 98
Copyright: Carolyn Ray

'For never will this be proved, that things that are not are' (saying of Parmenides, reported in Aristotle, Metaphysics 1089a3).

The ancient Greeks wrestled with the problem of how to say of the things that are not, that they are not. Contemporary identity theorists worry about how to say of the things that are not, that they are. I have asserted that logical possibilism is at the root of certain intractable puzzles in identity theory. I will now explore some specific implicit premises that logical possibilism and its peculiar method of thought experiments require, how they damage identity theory, and, if we decide to reject this method, what is to be done without its familiar and omnipotent tools.

Most identity theorists do not attempt to make a case for constructing science fiction thought experiments--a method I will refer to as 'sci-fi philosophy' or simply 'sci-fi.' The method is assumed to be valid, and it is generally not disputed (see Quine, Word; Wilkes, Real; and Williams, 'Personal,' for notable exceptions). Let us first consider some of the reasons that have been offered for doing sci-fi in identity theory, and see how they stand up to analysis.


Justification: When we do thought experiments, we are using an important tool for sharpening the boundaries of our concepts. Identity is sortally relative, in the sense that for any given identity statement of the form 'x is the same as y' we can ask, "Same what?" The 'what' refers to a concept or sort. So it is important that we know exactly where our concepts begin and end. Sci-fi thought experiments can help us with this because they force us to make the fine distinctions characteristic of a well-circumscribed concept.

Response: There must be some reason for sharpening concept boundaries, beyond the bare exercise. Traditionally, the reason for all philosophical activity has been to discover the truth. Whatever one's position on the nature and source of concepts, I take it as an uncontroversial point that of primary concern is the essential attributes associated with a given concept. To enable ourselves to state these attributes, we must examine the concretes to which to concept refers. But in sci-fi, the attributes of the concretes are invented, or, at best, left unspecified. But now it seems the project of boundary sharpening is not aimed at the truth about the world or our concepts, for it is not clear what connection these inventions are supposed to have to the world. One wonders whether sci-fi philosophers believe that the purpose of philosophy is to help us all understand reality, or whether they think it is a self-contained endeavor that need not concern itself with the troubles of common people.


Justification: Real life is messy and complicated. Using sci-fi thought experiments pares the issues down to their essentials. The stories could be better specified, but that would defeat the purpose of simplification.

Response: Sci-fi stories are presented in a clean, uncomplicated package that make it seem as though the issues are pared down to essentials. In fact, sci-fi examples are extremely complicated, whether the presentation of the story seems so or not. They involve (1) the suspension of disbelief, already a very complicated act of ignoring relevant background knowledge that contradicts the claims being made; (2) the invention of procedures and devices, often left up to the listener, that will achieve the effect about which we have suspended disbelief; (3) details about the subject(s) of the experiments, which are left out but must have effects nonetheless. Contrary to assertion, the sci-fi philosophers reason from the extremely complicated unknown, to the contradiction of the simple and well-established.

Hope For The Best, Prepare For Anything

Justification: Technology is exceeding the human capacity to keep up; and whereas science was once the proper realm of philosophers, it is getting away from us now. We need to stay ahead however we can. In the case of personal identity, for example, we can anticipate science. We know that hearts can be transplanted; it is just a matter of time before brains can be transplanted, and even divided and redistributed. How will our laws, for example, respond to all this body-switching? Clearly the ways we tell one person from another, and recognize them time after time, are inadequate to deal with all these advancements. Thus, the logical possibilities expressed through sci-fi help prepare us and our concepts for navigation even in a radically different reality.

Response: If we want to be ready for what is to come, the worst thing that we can do is clutter the field with imaginary beings. For example, if I am preparing for a road trip, it is rational to look into possible road construction, areas of high crime, and disreputable hotels along the route, even though I do not have positive proof that I will encounter these; it is not rational to prepare for encounters with aliens. If I simply invent the areas of concern, there are an infinite number of ways that I could go wrong; if I adhere to what I already know about reality, I am less likely to waste my time.

Furthermore, we have not yet dealt satisfactorily the questions of identity that now confront us--for example, when does a person come into being, what exactly could be meant by the term 'multiple personality,' and how can we establish criteria of identity for electrons--and the sci-fi philosophers show interest in these cases only to the extent that any anomalous behavior exhibited seems to justify the sci-fi method (see, e.g., Mills, 'Dividing,' and Kolak, 'Metaphysics'). If preparedness is really the motivation, it is more the purpose to speak as reasonably as possible about the things that we already know exist, so that we will have very clear procedures for adjusting our concepts should anything similar but slightly different happen along.

The Need To Respond

Justification: Admittedly, identity theory can probably be accomplished without sci-fi, but those who challenge our common sense concepts and philosophically defended working hypotheses are using sci-fi. The tone is therefore set by our opponents. Since our silence would imply our consent, we take the sci-fi seriously, and even invent our own logically possible scenarios to use against our opponents.

Response: Two points need to be distinguished: (1) It is questionable whether one must respond to challenges to theories about reality if the challenges are not about reality. Bernard Williams expresses this doubt (Williams, 'Personal'), but continues to answer in terms of logical possibilities. There are other ways of answering a sci-fi challenge, as we will see. Furthermore, when a philosopher of Williams's caliber and talent uses sci-fi even in the refutation of absurd propositions, it lends his authority to the method, and, when misinterpreted by the sci-fi philosophers, ends up providing more puzzles. (2) It is true that identity theorists sometimes accuse others of lying or of being mentally defective if they say that they cannot imagine the case, or that the example is not adequately specified. For example, Eugene Mills defends the use of a fission story:

Some philosophers profess failure of intuition in such cases and therefore would withhold judgment from [the claim that both products of a certain fission "operation" must be responsible for a crime committed by the original patient]. The case admittedly stands under described, but I believe it can be elaborated in obvious ways so that protestations of failed intuition would reasonably seem signs of insincerity or moral or mental defect (or eliminativism with respect to responsibility, which may or may not fall under one of the other headings) (Mills, 'Dividing,' p. 39).

However, arguments of this type illegitimately shift the burden of proof from the sci-fi philosophers to their opponents; this particular argument adds an appeal to ignorance and ad hominem . Mills does not attempt to elaborate his cases; he excuses himself from the responsibility of doing so, on the grounds that if his opponent cannot prove that the case is unimaginable, then it is not unimaginable.

Moreover, whether one can imagine the case is irrelevant to the truth of the conclusions drawn from it, so there is no point in arguing about imaginability. The real fault of the sci-fi stories is that they have nothing to do with reality, and therefore should not be taken to have any import for our theories or concepts about reality.

It's Scientific

Justification: Einstein invented incredible science fiction stories to prove his theories. Kekule came up with the ring structure of benzene through a ridiculous dream "thought experiment." If sci-fi is so useful in science, which is extremely successful at describing our real world, then it should be successful in philosophy too:

It may be impossible for some of these cases to occur, whatever progress may be made in science and technology. I distinguish two kinds of case. Some cases contravene the laws of nature. I call these deeply impossible. Other cases are merely technically impossible.

Does it matter if some imagined case would never be possible? This depends entirely on our question, or what we are trying to show. Even in science it can be worth considering deeply impossible cases. One example is Einstein's thought-experiment of asking what he would see if he could travel beside some beam of light at the speed of light. As this example shows, we need not restrict ourselves to considering only cases which are possible. But we should bear in mind that, depending on our question, impossibility may make some thought-experiment irrelevant (Parfit, Reasons, p. 219 [emphasis Parfit's]).

Response: Appealing to the authority of Einstein is a popular technique in the sci-fi genre of philosophy; here, Parfit is apparently referring to the thought experiment that can be found in Einstein's popular elementary tract, Relativity. But there is a vast difference between what Einstein used his thought experiment for, and what Parfit uses his for. Einstein did not use his thought experiment to show that, when traveling at normal speeds, we do not see what we think we see, or that things are other than they seem to be; and it was certainly not his starting point. He used it to familiarize mathematically unsophisticated people with the rudimentary principles of a theory that he formed on the basis of his mathematical calculations and theoretical knowledge--and all the empirical data that supported the theories. He did not use his imagination to conceive a scenario in accordance with his intuitions, construct a logically self-consistent statement, and then make up some math-like scribbles that might be taken as support for the statement. Nor did he ever hint that it might be possible for an observer to travel at the speed of light; he denies this possibility explicitly, and shows the math to prove it (Einstein, Relativity, Chpt. XII).

And it cannot be overemphasized that the theory Einstein was working with accounted for what we experience at normal speeds. Contrary to current ways of speaking in sci-fi circles, Einstein did not prove that classical mechanics was wrong about measurement at the speeds we experience; nor was that even his intention. Since this thought experiment is so often cited as the justification for identity theorists' contradictions of implicit and well-substantiated hypotheses, it is worth quoting the relevant text at length so that the matter can be put to rest:

....The epoch-making theoretical investigations of H. A. Lorentz on the electrodynamical and optical phenomena connected with moving bodies show that experience in this domain leads conclusively to a theory of electromagnetic phenomena, of which the law of the constancy of the velocity of light in vacuo is a necessary consequence. Prominent theoretical physicists were therefore more inclined to reject the principle of relativity, in spite of the fact that no empirical data had been found which were contradictory to this principle.

At this juncture the theory of relativity entered the arena. As a result of an analysis of the physical conceptions of time and space, it became evident that in reality there is not the least incompatibility between the principle of relativity and the law of propagation of light, and that by systematically holding fast to both these laws a logically rigid theory could be arrived at. This theory has been called the special theory of relativity...(Einstein, ibid, Chpt VII [emphasis Einstein's]).

Classical mechanics required to be modified before it could come into line with the demands of the special theory of relativity. For the main part, however, this modification affects only the laws for rapid motions, in which the velocities of matter v are not very small as compared with the velocity of light. We have experience of such rapid motions only in the case of electrons and ions; for other motions the variations from the laws of classical mechanics are too small to make themselves evident in practice (Einstein, ibid, Chpt XV [emphasis mine]).

Parfit's variations from our ordinary understanding of ourselves, however, are quite evident in practice. In addition, he skips the step in which a highly specified theory is supported by lots of empirical data. Instead, he begins with a vague notion of persons; uses his imagination and confirms the resulting claims to be logically possible; and then concludes from this that what we think we are in the normal case is wrong. If there is any merit to the type of thought experiment that Parfit engages in, it cannot be shown by comparison to Einstein's thought experiment, since there is little similarity.

If nothing hinged on Parfit's sci-fi counterexamples, if they were simply fun exercises to be engaged in in the same way that one enjoys a science fiction movie, there would be no reason to dispute the issue. But Parfit's ultimate question is what ethical theory is correct, given the nature of persons; and his experiments allegedly show that persons are drastically different than they seem in the normal case. This leads us to the last justification we will consider.

Real Human Beings Can't Help Me Prove My Ethical Theory

Justification: It is difficult to show that my ethical theories are correct if I am restricted to the ordinary, provincial conception of persons. Since it is logically possible that persons are very different than they apparently are, my ethical theory is true, since the theory takes as its referents this different sort of being.

Response: Obviously, it would be a rare person indeed who would own up to such a motivation. Yet this seems to be the motivation for Parfit (see also Perry, "Importance," and Unger, Identity). From the sci-fi, Parfit concludes that all that can rationally matter to one is that one's pet projects be completed, and it does not matter who completes them; further, one should consider one's replacement by an exact replica to be not nearly so bad as death. Therefore, the ethical boundaries between persons are not what we thought they were.

Scientific and introspective data is ignored not because it would contradict the metaphysical theory being propounded, but because it contradicts the desired ethical conclusions. Because an ethical code is grounded and worthwhile only to the extent that it takes into account the nature of the creatures it is intended to guide, the nature of the creatures must be established antecedent to any ethical code, and that code must be based upon data about those creatures. In other words, basic empirical data, epistemology, and metaphysics must be worked out first, and then upon these the ethical code can be developed. To dismiss scientific data because it contradicts an ethical code is tantamount to embracing subjectivism.

Intermediate Observations Regarding Logical Possibilism

The basic structure of the arguments from logical possibilism can now be understood. The structure is this:

1. If circumstance x obtains, then theory T is true.

2. We cannot show that x obtains.

3. But we will show that x is possible by means of a thought experiment.

4. Consider the proposition describing x, namely, 'x'.

5. If 'x' is not self-contradictory, then x is possible.

6. But of course 'x' is not self-contradictory.

7. Therefore, x is possible.

8. So T is true.

Showing that a proposition is not self-contradictory cannot show that a theory based upon it is true. But there is more here than at first meets the eye. There is one and only one way that x can be shown to be impossible, as far as the logical possibilist is concerned: it must be shown to be self-contradictory. This means that there is no way to show that x is impossible. Why? Because it has been assumed to be possible, by the very doctrine of logical possibility. Possibility is, if you will, contained in any proposition that is not self-contradictory. Thus, the question has been begged. To put it another way: the step from 5 and 6 to 7 looks like an informative inference, but it is not. Proposition 5, in accordance with the rules of logical possibilism, really says,

If x is possible, then x is possible.

Now of course, the proposition 'x is possible' follows from this statement, but only because the form of the argument is, 'if x, then x; x; therefore, x.' Deductive, to be sure, but circular.

In addition, our interlocutors were supposed to be showing that theory T is true. Allegedly, x's being possible would be sufficient for the truth of T. The logical possibilist will admit, if pushed, that all that is meant by x's being possible is that 'x' is not self-contradictory. This is not what we need to get the truth of a theory; we need real possibility. So the only way to get T from the non-self-contradictoriness of 'x' is to equivocate on 'possible.'

There is more. Since it would take nothing short of self-contradiction to show that x is not possible, the implicit premise is, 'if you cannot prove that it is impossible, then it is possible.' This is a textbook case of the appeal to ignorance, a popular but worthless argument form. What we need to claim that x is possible is some bit of evidence that it is. Our being unable to disprove that it is not possible does not count as evidence, nor is it our responsibility to look for evidence against x.

Finally, logical possibilism reduces the method of reductio ad absurdum to absurdity. For logical possibilism, in the sci-fi genre at least, the only absurd proposition is a self-contradictory one. But it is unlikely that a philosopher would ever attempt to prove a self-contradictory statement. For example, try to imagine someone attempting to prove 'Persons are not what they are.' A philosopher will only begin to talk about logical possibility if he or she has already weeded the contradictions out of the proposition, so that it takes a form such as, 'Persons might be different, in some possible world, than they are in this world,' which is not blatantly self-contradictory. The proposition, 'Sea-monsters frequently speed on I-70' is similarly not self-contradictory. Hence, the reductio ad absurdum of logical possibility reduces to absurdity, because it grants immunity to affirmations of nonsense.

It might be objected that this is not what identity theorists are doing, that they do not countenance just any proposition as possible. For example,

Now, of course--someone might object--I can imagine being a telephone. Does that show that what I take to be boundaries between myself and the telephone are illusory, or that it is possible that I am the telephone? No. Ceasing to be this particular human being and becoming a telephone would not, we believe, preserve what matters to my continued existence. Telephones cannot think (not yet anyway), cannot feel, reason, move about; a telephone could not finish this article. Where as if my living brain were put into your living body and continued to function as it now does, then, we can believe, I would have a new body and using the new body I--the very person now writing these words--could finish this article; my personal identity, it seems, could be preserved across that border....(Kolak, 'Metaphysics,' p. 47 [emphasis mine]).

But then the question immediately arises, how is this person drawing his imaginability boundaries? It is no more unimaginable that a telephone could finish that article, than that Kolak's brain could continue writing that article using my body. In any event, even if the accusation of full-blown logical possibilism cannot be made in some cases, we can see that the appeal to ignorance is still the operative premise: we, Kolak's opponents, cannot prove that brain transplants are unimaginable; therefore, they are not unimaginable. And if they are not unimaginable, then they are possible. And if they are possible, we might as well go ahead and believe his theory T.

When we look at it this way, sci-fi seems awfully silly. But maybe this evaluation is not completely fair. There must be some reason that identity theorists have tended to rely on fantasy. Is there any justification for fantasies? If so, what kind?

Valid Thought Experiments

Consider the following thought experiment that uses fantastic objects and yet is valid. Suppose that you are in a boat in a pool, and there is a brick in the boat. If you throw the brick out of the boat and into the water, does the water level in the pool rise or fall? To easily see that the water level will fall, exaggerate the measurements of the brick and let the other conditions remain constant. Suppose that the brick is made of some extremely dense material, so that it threatens to submerge the boat, but that it is only the size of a golf ball. The golf ball is pushing the boat into the water, which raises the water level. Throwing the ball from the boat will allow the boat to rise out of the water, lowering the water level. But the ball by itself is so tiny that it cannot take up much room in the pool, so although the ball would make the level of any body of water rise a little, it cannot make it rise as much as it can when it is pushing down the boat. Now reintroduce the measurements of the brick. The brick makes the boat sink a little. Throw the brick into the water. The brick will not take up much room in the pool, but it will take up a little. Moreover, the brick made the boat sink by a certain amount when it was in it, and now that influence is gone, so the boat rises a little, allowing the water level to fall. The water level rises on account of the brick, but falls on account of the boat getting up out of the water. Because the boat takes up more space than the brick, the boat displaces more water with the brick in it than the brick would on its own. So when the brick is thrown overboard, the water level falls.

What is important to note about this thought experiment is that, though it is not possible for a golf ball to nearly sink a boat, nothing hangs on that fact. Exaggerating the measurements is a heuristic device intended to focus attention on the salient features of the problem. The exaggeration is not intended to contradict our knowledge about boats, bricks, or pools.

What Counts As A Counterexample?

Identity theorists frequently portray themselves as presenting counterexamples to our common sense or philosophically-established theories; this method is supposed to be legitimate because this is exactly how scientists test their theories, and, as Karl Popper has told us, the mark of a bad "scientific" theory is that it is not susceptible to falsification by counterexample.

For scientists, a theory is a set of statements about the real world. If a theory is to be shown false, then either an internal inconsistency among the statements of the theory must be revealed, or a counterexample must be produced. A counterexample can come in the form of a true statement about the real world that contradicts the statement(s) of the theory, or in the form of a demonstration or ostention .

A fictional statement describes a state of affairs that does not obtain, whether it could obtain or not. Counterfactuals are a subcategory of fictional statements. A counterfactual is a statement that describes a state of affairs that could obtain in the world, but does not. For example, I might say, "Imagine that my grandmother has succumbed to Alzheimer's disease--is she still a person by your criterion?" or "Suppose that I have become paralyzed from the neck down--am I still embodied in the relevant sense?" Counterexamples need not describe an actual occurrence in the real world (my grandmother need not have Alzheimer's disease), but only that which we know by experience to be the sort of thing that could occur (Alzheimer's disease is something that happens to human beings, and my grandmother is human). Counterfactuals thus can be used to in the formulation of counterexamples.

Science fiction is a subcategory of fictional statements too; this subcategory includes statements that describe states of affairs that do not obtain in the world; some of these we know can obtain, but we do not know how to make it so; others we know cannot possibly obtain; still others we do not know about. Sci-fi philosophy's error is to use logical possibility as a guide to imaginability, and in turn to use imaginability as a guide to developing counterexamples.

Identity Theory's Thought Experiments

A valid thought experiment, then, is one which describes the essential features of a problem, abstracting away from the measurements but eventually reintroducing them. A valid counterexample, which can be used in a valid thought experiment, tests or contradicts a theory by means of citing actual or possible states of affairs. But identity thought experiments often take this approach: no measurements are considered, except to say that they do not matter, and proposed counterexamples consist of scenarios that we either do not know are possible, or that we know are not possible. Consider the typical brain transplant identity puzzle. Facing the threat of objection on the grounds that the procedure is not known to be possible--and, due to the complexity of the brain and the nature of persons, is very likely not possible--the first thing that the sci-fi philosopher premises is that there is no difference between the two persons considered. As this is not readily apparent in the descriptions of the cases, an example will illustrate what I mean.

Suppose that the experiment posits two persons who are very similar in their bodily appearance and makeup (twin sisters, say) and that the brain of one is to be put into the body of the other. It may be objected that the body grows and changes to accommodate new experiences and tastes, etc.; having had different histories, the sisters will have different nervous structures and therefore the transplant will not be possible. The reply quickly comes that the two have extremely similar histories as well as bodies, and that this similitude will permit the transplant. Now it can be insisted that the sisters must have had different interests, or at least must have known different people; so it can still be claimed that the nervous structures are too different. Then the experimenter may change the initial conditions to include sisters who spent all of their time in close proximity, as twins sometimes (almost) do, and thus they knew all the same people and had the same interests. When one objects that the close twins in one's own experience have had different interests, insights and perspectives on things despite their inseparability, the experimenter will simply posit those differences out of existence: "Just suppose that they are alike in all of their tastes, interests, insights, perspectives, motives, etc., as well as in their experiences, histories, bodies, and nervous structures."

We are entitled to object at this point that the problem of personal identity (i.e., how to know which person is which and which person has continued and which has not) did not arise as a result of the transplant, but as a part of the initial conditions. In other words, we started out completely unable to tell one twin from another (and we can only presume that they did too, since if they could distinguish themselves then there would be differences in their experiences), and we did not need the transplant in order to challenge our common sense notions in regard to personal identity. In the initial setup we are asked to grant that there is a difference between these two things, other than relative physical position, which means that they will measure differently against at least one standard, but we are told for every standard we can think of that there is no difference in measurement. The initial conditions are thus self-contradictory. If there really is no difference between these two women, then we started out with only one woman. This is, I take it, the meaning of the "identity of indiscernibles." And now we must draw the unhappy conclusion that, if pushed far enough with questions about details, the sci-fi logical possibilist will simply deny Leibniz's Law. What is to be gained by this transgression is unclear.

In a valid thought experiment, the differences are enhanced in order to bring out the important features of the problem. In a sci-fi thought experiment, the differences are ignored. In a valid thought experiment, the measurements that were omitted are eventually reintroduced to establish the bearing the experiment has on reality. In a sci-fi thought experiment, measurements are never taken, and the suggestion that they be taken is considered evidence of intellectual dishonesty. Legitimate thought experiments encourage probing of the issue; illegitimate thought experiments require suspension of thought.

Finally, I contend that sci-fi "establishes" theories by violating the very falsifiability criterion that identity theorists hold dear. How? Theories and hypotheses are established on the basis of a non-self-contradicting proposition. Since, to any objection to the theory based on the real world, the logical possibilist will say, "There's no logical necessity that the world has to be that way--it could be this other way," there is no circumstance that could disprove the theory. Since non-self-contradictoriness is the only criteria logical possibilism recognizes, all theories are good theories.

What Is A Valid Philosophical Thought Experiment Like?

Philosophical thought experiments are useful insofar as they make obscure principles clear, so that the essentials of the case may be grasped more easily. Consider John Locke's brief thought experiment:

...if the consciousness went along with the little finger when it was cut off, that would be the same self which was concerned for the whole body yesterday, as making part of itself, whose actions then it cannot but admit as its own now. Though, if the same body should still live, and immediately from the separation of the little finger have its own peculiar consciousness, whereof the little finger knew nothing, it would not at all be concerned for it, as a part of itself, or could own any of its actions, or have any of them imputed to him.

This may show us wherein person identity consists: not in the identity of substance, but, as I have said, in the identity of consciousness, wherein if Socrates and the present mayor of Queinborough agree, they are the same person.... (Locke, Essay, II, xxvii, 18-19).

Locke uses this experiment to focus attention on the fact that our normal ideas about a person include first and foremost its psychological features and that it is appropriate to disregard the loss of a body part. He does not use it to reach the conclusion that we were all the while mistaken about persons, and they really are such that they could be moved into the little finger. A rather wild scenario is introduced to show the force of a hypothesis that is, if not established, at least not incredible; but nothing hangs on the scenario itself. By contrast, for modern identity theorists, that wild scenario is the hypothesis; and moreover, by the very form of the proposition describing it--'This person once ruled an entire body, but is now restricted to governing only a little finger'--they take it to be both credible and possible.

Real Life Is Puzzling Enough

The real cases and issues are often very messy but philosophically more interesting in the end: the quandaries are real, and the answers are important to someone. There is no obligation to respond to sci-fi counterexamples; however, there is a great deal to be gained by comparing the science fiction to the real science it is alleged to take as its point of departure. It is useful to see what sort of data is being made up, what sort of scientific data is being ignored, and how the data is being manipulated to fit the particular theory of identity.

Brain Transplants

Consider first the commissurotomy--or split-brain--patients, whose cerebral hemispheres apparently respond independently when one hemisphere of the brain is fed stimuli which does not reach the other hemisphere through the corpus callosum. The conjecture that one hemisphere may be removed ignores important information relevant to the study not only of personal identity, but to the identity of non-persons as well. The assumptions needed to make this conjecture seem plausible are (1) that the relationship between the hemispheres as facilitated by the lower portions of the brain and by the feedback provided by normal sensory perception and subsequent conceptual processing is irrelevant to the proper functioning of each hemisphere; (2) that a cerebral hemisphere is like a suitcase, and as such contains and carries with it everything a person would need for a trip to a new body; and (3) that human bodies are similar enough so that the operation can be performed successfully and with a minimum of disorientation--hearts and kidneys are transplanted; why not brains?

Roger Sperry, the neurologist who discovered the effect of commissurotomy, made some claims that are philosophically controversial; he spoke of the commissurotomy "dividing the mind" and of "two minds running in parallel" (Trevarthen, 'Brain'). However, as might be expected of a non-philosopher dealing with hitherto undiscovered and very surprising phenomena, neither he nor his more skeptical colleague, Colwyn Trevarthen, is very careful about defining terms and using them consistently. Clarifying such terms, given the scientific context, is the real job of the philosopher, and this is where the sci-fi identity theorists fail to perform their duties. The data on brain-damaged human beings suggest that the loss of even a small part of a cerebral hemisphere can cause immense problems of coordination, understanding, self-awareness, and familiarity with oneself (i.e., accurate recall of one's distant history and the integration thereof with one's current and continuing experiences). The Sperry experiments also show that, far from becoming the material substrata of independent minds, the hemispheres bypass the defunct corpus callosum and stay in direct contact with one another by means of peripheral vision, orienting movements of the eyes, feedback from the ears, proprioception, and other feedback mechanisms.

Furthermore, it is not the whole brain but the cerebrum, one part of the brain, which has some but not all of its connections severed. Both hemispheres are still connected as normal through the brain stem and are thus exchanging information through it, if at a slower rate. It is true that some of the connections between some parts of the brain can be severed without killing the human being. It is also true that the human being then behaves, when under highly artificial experimental conditions, as though unaware of a stimulus fed to only one hemisphere while nevertheless responding to that stimulus. But these two facts do not warrant the conclusion that we can in principle transplant a hemisphere with "a little tampering" and make a second person from one of the hemispheres. I will return to this issue in detail in the chapter on personal identity.

Partial Gradual Replacement?

Alzheimer's disease and multi-infarct dementia give us two good reasons for thinking that partial gradual replacement of brain cells would, at most, result in the termination of both the donor and the recipient. This is a fairly strong claim, but I think there is good evidence for it.

Recall Peter Unger's sci-fi thought experiment, mentioned in the catalog of puzzles, involving the gradual replacement of his brain-cells with those of Einstein. Unger expects that Einstein will gradually show up in Unger's body. But a brief introduction to the cause of the disability known as Alzheimer's disease will show what is wrong with this notion. Alzheimer's disease is caused by wide-spread or localized loss of nerve-cells in the cerebral cortex, or outer layer of the cerebrum; the greater the loss of brain cells, the greater the loss of mental function. To elaborate, what is happening to the Alzheimer's brain is gradual loss of brain cells; this leads to the loss of specific memories, of the ability to remember, and of the ability to relate to the environment. The last things to go are usually personality traits and social skills; the first is often memory--both distant memories, and the ability to remember at any given moment what one is doing.

In light of the interest in various sorts of brain division and the accompanying fear that we will not be able to tell one brain-part recipient from another, these facts are significant to the study of personal identity. When Unger asks us to imagine that someone's brain cells are being gradually siphoned off, he is essentially asking us to imagine a victim of Alzheimer's disease. Focus attention for a moment on just the part of the brain left in the victim/donor once memory has begun to be lost: this left-over part is large, especially in comparison to the amount of brain cells that have been drained off, and the person is still functioning but suffering frequent bouts of forgetfulness and failure to remember the past; the person is now considered demented. If memory is lost while so much of the brain remains, what chance does any trait of the donor, including memories, have of being in those brain cells which have been removed? And if they are not in the remainder of the brain, nor in the cells, where are they? Given a few unsensational facts, the experiment deteriorates into incoherence. Moreover, we appear to have uncovered, if not a commitment to, at least a requirement for, dualism, since the only way that those memories and the faculty of memory can get themselves into Unger's brain is if they need not be in any physical substrate at all.

In addition, Unger's thought experiment requires the assumption that, at the cellular level, one brain cell is just like any other. This assumption contradicts the fact that neurons grow, and produce specific amounts of hormones and respond at specific rates given one's history, interests, skills, and knowledge. In fact, it contradicts his very project, which assumes that brains are different at the cellular level and that that difference underlies the differences in memories, inclinations, and personalities of Unger and Einstein. Unger might be tempted to introduce a sci-fi technique to eliminate these differences--but to do so would be to eliminate either Einstein or Unger, or both, since neither is a clean slate.

On that note, I wash my hands of sci-fi. There is no reason whatever to devote energy to reconsideration of a given concept on the basis of propositions whose only excuse for existence is that their words do not contradict each other in an obvious way. The rest of this work will take the matter to be closed. Further mention of sci-fi will only occur where there is some additional reason to consider a given puzzle or criterion of identity.

Concluding Remarks

We have seen that it is possible to construct good thought experiments, and even to use wildly inaccurate scenarios, to help to bring a difficult set of hypotheses within the range of the average person's understanding. But we have also seen good reason not to redefine our terms every time a science fiction puzzle contradicts them. If the hypothesis proposed cannot account for the concepts and identity criteria we use successfully to navigate the world, then it is illegitimate.

Philosophers need not suffer without logical possibilism and the science fiction thought experiments to which it lends its authority. Data from the sciences provide plenty of examples for us to contemplate and sharpen our definitions of what it takes to be a being and a persistent being of a certain type.

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