Harry Cleeveley

Physicalism and externalism are widely held theories about mind. Physicalism is the thesis that the physical properties of the world are sufficient for consciousness; externalism is the claim that certain external objects are necessary for some mental contents. Although these two theories are independent, they do have some important features in common. I will argue that both are false for the same underlying reason, which is that they entail false claims about the space of possible worlds. Physicalism and externalism can both be formulated in such a way as to rule out certain kinds of possible world. But, in both cases, we can use conceivability arguments to show that the worlds in question are in fact possible, and therefore that these doctrines are false. The most famous such conceivability argument is Chalmers’s (2010) two-dimensional argument against physicalism; along the same lines, I will set out a two-dimensional argument against externalism. These conceivability arguments can only succeed if ideal conceivability entails metaphysical possibility. But does it? Since the work of Kripke (1971), the philosophical orthodoxy is that it does not. This is because it is widely held that there are a posteriori necessities – that is, there are propositions that are true in every metaphysically possible world, but whose truth cannot be known on the basis of rational reflection alone. The negation of such a truth is therefore conceivable, but not possible. But this view is not universal. Chalmers (2010) argues that the Kripke cases trade on an ambiguity in the referring expressions they contain. Thus they have one sense in which they are necessary, and another sense in which they are a posteriori, but no sense in which they are both necessary and a posteriori. If this analysis is right, then the Kripke cases do not in fact disprove the entailment from conceivability to metaphysical possibility. So the fundamental question is whether there are any strong necessities – which are necessary and a posteriori on the same interpretation. Many examples have been proposed; much debate revolves around whether or not these supposed examples are compelling. In this thesis I will argue that there are in fact no strong a posteriori necessities at all, because they are impossible – the very idea, in fact, is incoherent. Therefore I conclude, against the conventional wisdom, that ideal conceivability does entail metaphysical possibility. This, in turn, leads to an internalist and anti-materialist metaphysics of mind.

There are good reasons to think that I am a Lockean person, and there are good reasons to think that I am an animal. But there is a problem: an animal cannot be identical to a Lockean person, as the two types of entity have different persistence conditions. And so the standard response of Lockeans has been to reject the claim that I am, strictly speaking, an animal. Similarly, animalists, while they can accept that we are persons in a broader sense, deny that we are Lockean persons. I find this unsatisfactory. Just as I think it is true of me that I would survive the teleporter, so I think it is true of me that I would continue to exist in a persistent vegetative state. But how can this be so? The answer, I argue, is that there are two thinkers at my location, a Lockean person and a human animal, and that my use of the first-person pronoun is ambiguous between these two entities. Thus there is one sense in which I am a Lockean person, and would survive teleportation, and another sense in which I am an animal. I also show how my proposal avoids the problem of there being too many thinkers at my location.

Let _P_ represent the totality of the fundamental physical truths, and let _M_ represent the totality of non-fundamental physical truths. What is the relationship between P and M? In this paper, I argue for two related claims. The first, which I term the _inscrutability thesis_, is that there is no a priori entailment from P to M. This means that even a God-like intellect, given all the truths of P, could not deduce that M obtains without further a posteriori information. The second claim is that P alone is not metaphysically sufficient for M– or, to put it another way, that M does not supervene metaphysically on P. Rather, I argue, M supervenes on the combination of P with the totality of truths about phenomenal consciousness, represented as _Q_. I call this thesis _second-order idealism_. According to second-order idealism, if Q had been sufficiently different, then M would have been different– even if we hold P constant, and even assuming the mind-independence of P. Thus the higher-order structure of the physical world depends in part upon consciousness.

Modal rationalism is the claim that for all p, if it is ideally conceivable that p, then there is a metaphysically possible world, W, in which p is true. This will be true just if there are no strong a posteriori necessities, where a strong necessity (for short) is a proposition that is conceivably false, but which is true in all metaphysically possible worlds. But are there any strong necessities? Various alleged examples have been proposed and argued over in the literature, but there is no consensus on whether any is genuine. In this paper, I aim to move the debate forwards by proving that there are in fact no strong necessities. I argue that there are no strong necessities because they are impossible; they are impossible because the very notion of a strong necessity is – despite prima facie appearances – ultimately incoherent. Thus, I argue, it is an a priori truth that there are no strong necessities, and that modal rationalism itself is true.

Modal rationalism is the claim that for any proposition p, if it is ideally conceivable that p, then there is a metaphysically possible world, W, in which p is true. If true, modal rationalism must itself be an a priori truth. Moreover, modal rationalism is true just if there are no strong a posteriori necessities. But are there any strong necessities? In this paper, I set out a transcendental argument to show that there cannot be any, because they are not genuinely conceivable. I argue that if it were ideally conceivable that there are strong necessities, then it would be impossible for us to have any modal knowledge of the world. Given that we do have some modal knowledge of the world, it follows that strong necessities are not ideally conceivable and therefore that modal rationalism itself is an a priori truth.

The zombie argument against physicalism (Chalmers 1996, 2010), if successful, shows that one of two things must be true: either zombies are possible (and therefore physicalism is false) or some form of panpsychism is true. Moreover, this is generally understood as an exclusive or: the truth of panpsychism at the actual world would be incompatible with the possibility of zombies. But is the mere possibility of panpsychism incompatible with the possibility of zombies? Giberman (2015) argues that it is, and that zombies are not really possible. Giberman’s anti-zombie argument involves what he terms the mereological threshold of consciousness (MTC). The MTC is, roughly, a threshold of complexity that must be crossed for a physical system (such as a human being) to achieve consciousness. According to Giberman, this presents a dilemma: either the MTC could not have been otherwise, in which case zombies are not metaphysically possible; or it could have been otherwise, in which case panpsychism is 1-possible. But, he argues, the 1-possibility of panpsychism is incompatible with the 1-conceivability – and, a fortiori, the 1-possibility – of zombies. Either way, zombies are not really possible. The aim of this paper is to show the MTC argument fails. I agree with Giberman that, if zombies are to be metaphysically possible, then it must be the case that the MTC could have been otherwise; and I agree that this entails that panpsychism is 1-possible. However, I argue that the 1-possibility of panpsychism is compatible with the 1-possibility (and indeed the 2-possibility) of zombies. It is only the 2-possibility of panpsychism that would present a problem for the zombie argument. But the zombist can quite comfortably reject the 2-possibility of panpsychism, as this is not entailed by the MTC considerations.