C ORRIENTES EPISTEMOLÓGICAS PREDOMINANTES EN EL PENSAMIENTO
7.13. T EORÍA DE LA ARGUMENTACIÓN JURÍDICA (TAJ)
In a survey article on the identity of indiscernibles, Forrest (2012) suggests that the identity of indiscernibles appeals to empiricists, for it seems we could never have any evidence that indiscernible items are distinct. We may have evidence that intrinsic duplicates exist, but in order to recognise that they are distinct, they must somehow be differently related to us: in order for someone to be able to count two of them, one must have a property that the other lacks, such as being to the left of the person doing the counting. Coupling this with the premise “that there are no two things which are not empirically distinguishable”, we reach the conclusion that the identity of indiscernibles holds (Forrest 2012, Section 3 (i)).
In the following chapter I will defend the principle that, if we consider that there is a possible world in which the observable facts make it objectively reasonable to conclude that p, then we should say that p is possible. Interestingly, Forrest also seems to rely on such a principle in his discussion of the empiricist argument against the actual truth of the identity of
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empirically distinguishable” (Forrest 2012, Section 3(i)), Forrest goes on to say the following:
Presumably the premiss would not be proposed as anything more than contingently true. For there are possible situations in which there would be theoretical reasons for believing in indiscernible items as a consequence of a theory which best explains the empirical data. Thus we might come to hold a theory of the origins of the physical universe which had large amounts of empirical support, and which implied that, in addition to our enormously complicated universe, various simpler ones had been generated. For some of the simplest universes this theory might imply that there were exact replicas. (Forrest 2012, Section 3(i))
Forrest is here appealing to hypothetical evidence to cast doubt on the empiricist’s premise; but we may have actual evidence to do the same job, for it has been argued that certain phenomena of quantum physics are best understood in such a way that implies the existence of distinct but indiscernible objects. French (1989) shows that, if we are allowed to identify state-dependent properties of particles “with all the monadic and relational properties which can be expressed in terms of physical magnitudes that can be defined for the individual particles,” a suitably weak version of PII is violated in quantum mechanics, for “two bosons or two fermions in the appropriate superposition state have the same monadic properties and the same relational properties one to another” (French 1989, 158).
This has not gone unchallenged. Saunders (2006), for example, points out that, while it is indeterminate which of a pair of maximally entangled fermions is spinning in which
direction, it is determinate that each is spinning in the opposite direction to the other, and that neither is spinning in the opposite direction to itself. This means maximally entangled
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allow PII to incorporate such relations, the result is that maximally entangled fermions, while distinct, do not violate the principle – they are, after all, (weakly) discernible. As for
entangled bosons, Saunders maintains PII by claiming that “we would do better to say that there is only a single particle present (with proportionately greater mass)” (Saunders 2006, 60). Others have offered alternative – albeit non-standard – interpretations of quantum mechanics, which allow PII to be retained (van Fraassen 1985 and 1991).
Thus the dispute over the significance of quantum physics for PII is a matter of some controversy, and not one that I am able to adjudicate. But at the very least, we can note a presupposition of that debate: best theories of actual contingent phenomena can be a guide to possibility. Indeed, the very effort to show that the account of quantum phenomena that entails the falsity of PII is not the best account seems to presuppose that, had it been the best account, we would have had grounds for denying PII.47 So what about best theories of merely possible but contingent phenomena? In the next chapter I explore the idea that the best accounts of possible phenomena should themselves be taken to be possibly true. In this way, possible evidence can be a guide to possibility. More precisely, when we consider that there is a possible world in which the observable facts make it objectively reasonable to conclude that p, we should conclude that p is possibly true. This seems to be the principle behind Shoemaker’s (1979) argument for the possibility of time without change. In the next chapter, I’ll use that argument to show how the principle can provide a way of arguing for a certain possibility claim where there are dialectical constraints on the probative force of imagination.
47 Matters are complicated, of course, by the fact that some will take a theory’s implication of the existence of
distinct indiscernibles as a reason for rejecting that theory (one person’s modus ponens is another’s modus tollens). Given his long-term commitment to an empiricist/anti-scientific realism stance, and the fact that an empiricist stance can be taken to favour PII, this is presumably at least part of what drives van Fraassen’s alternative interpretation of quantum physics. However, as noted in my Introduction, I am taking scientific realism for granted.
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