In Chapter Six, especially section 4 , 1 consider how important the differences between fission and ordinary survival are.

change from our actual world. Fission, brought about by operation, again only requires that certain things are possible in the operating theatre that are not so now. The combined spectrum and teletransportation cases require much more technological development. But even here, to use the experiment, we need only imagine one thing being different from our current world - that the technology exists to perform these operations. These do indeed seem to entail the alteration of one constant and one constant only which Wilkes demands of thought experiments. (But see the second objection below).

Wilkes’ and Rovane’s objections sound persuasive because they ask us to consider the wider social and personal differences that a world in which we divided, teletransported and who knows what else, would entail. But in all four thought experiments, there need be no consequences for society or people in general. All four can be conceived as one-off possibilities for single individuals. We could imagine that two scientists have been working on these technologies in a private laboratory and only performed the experiments on themselves. The question of their identity over time would still arise even though they lived and grew up in a society where such changes are alien.

So, if Parfit asks “what if I was teletransported?’’ there is no requirement for him to spell out the wider implications of this weird technology on society for the question is equally valid when applied to the first and maybe only person to use the device.

There is, however, still a sense in which there is room for doubt over the background conditions. We might wonder whether the laws of physics have to be changed in order for us to imagine these cases. If they would, then we might not have a case of just one fact about the world being altered but something more like “several factors all fluctuating". This thought leads onto the second type of objection, based on considerations of imagination and possibility.

Again, Wilkes is the clearest on this issue. She first considers different types of possibility. First, there is logical possibility. Anything which doesn’t entail any

contradiction is logically possible, such as pigs that fly, or sheep living on Mars. But this puts only a small constraint on what is possible. Most philosophers do not accept that everything that is logically possible is actually, or in Wilkes’ terminology, ‘theoretically’ or ‘in principle’ possible. This could also be called physical or nomological possibility, but I shall stick to Wilkes’ terminology. Take, for example, the case of iron floating in water. There is certainly nothing contradictory about this, but with all we know about iron and water, many would like to argue that this is not in fact a real possibility. Although we can say we can imagine iron floating on water, this is not enough to make this possible. Either what we are imagining is something like iron floating on (something like) water or imagination is simply no guide to possibility at all.

These issues of imaginability and possibility are complex and involve considerations of meaning and reference, externalism and internalism that neither Wilkes nor I have given much time to. Certainly, for Wilkes’ point to be truly conclusive we have to take a lot of other philosophy on faith. But Wilkes doesn’t depend upon us reaching any firm conclusions about this. Indeed, the very contentiousness of these issues works in her favour. Her point is really that any thought experiment which does not work within the parameters of what is known by the physical sciences is open to the charge of depending on an impossibility and thus being at best inconclusive and at worst, irrelevant. For example, a thought experiment that requires us to hold that pigs can fly may not to convince us of very much. Whether or not we decide this a real possibility or not, the charge can still be made that this is irrelevant because in our world pigs can’t fly.

In essence, this point is an extension of the first criticism about specifying background conditions. Here, the laws of physics are considered relevant background conditions. If a thought experiment flouts these laws, then we may

not learn much from iti?, as there Is no single factor being altered against the constant background which any form of experimentation requires. An example of this sort of objection at work can be seen in John Robinson’s critique of Parfit’s argument from fission, who claims:

Recent experimental work by Colwyn Trevarthen and his associates suggests that such a conclusion [there are two streams of consciousness in commisurotomy patients] is incorrect.

Here, it is taken as read that establishing the actual possibility of the phenomena of divided consciousness is important for the thought experiment. Parfit is ambivalent about this. Although he uses the commisurotomy experiments to refute the objection that divided consciousness just isn’t possiblei9, he also says “it can be useful to consider impossible thought- experiments.’’20

In order to clear up the issue of the relevance of possibility to thought experiments, we first need to get clear on the different sorts of possibility. We have already seen Wilkes' distinction between logical and theoretical possibility. Let us now turn to Parfit’s conceptions. Parfit distinguishes between that which is “deeply impossible" and “mere technical impossibility” .21 In the

fission case, it may be technically impossible to divide the brain, as perhaps the brain stem cannot be cut or divided. But according to Parfit, what is still deeply possible is that consciousness can take a dividing form. In both cases, empirical data can be important. The importance of the commisurotomy cases is that it shows something which might have been deeply impossible to be possible. As

17 It is possible that a thought experiment might require the laws of physics to be broken, if this is