¹ Locke, John, An Essay Concerning Human Understanding, ed. Nidditch, P. (Oxford, Oxford University Press, 1690/1975), Book II, chapter 1, §10.Google Scholar

² Peacocke holds that a subject of experience x is identical with a subject y if and only if x and y have the same S-generator – or in his terms ‘same material integrating apparatus’ (The Mirror of the World, 66). But he does not claim that a subject is numerically identical with an integrating apparatus, since the latter possess properties (such as being spatially localized in the brain) that the subject does not – he takes subjects to be metaphysically basic entities. But for present purposes what matters is that he does hold that ‘the identity of a subject over time consists in the identity of the apparatus that integrates states and events in such a way that a single subject has, or may have, perceptions, sensations, thoughts, action-awareness, and the rest, both at a time and over time.’ (Ibid., 65)

³ Achieving an instantaneous change of parts would be impractical using anything resembling standard surgical techniques, but this does not mean it is physically impossible, just rather improbable. Cosmologists have recently started worrying that the most numerous conscious subjects in the universe might be Boltzmann brains, i.e. brain-like physical systems that are produced by random quantum fluctuations; these systems only enjoy a brief period of experience: shortly after their creation they vanish again, as their particles disperse. The spontaneous creation of experience-sustaining brains is immensely improbable, but if the universe is infinite – as currently seems likely – there will nonetheless be vast numbers of these entities overall. If Boltzmann brains are physically possible, then there is no obvious reason why these same processes couldn't cause a brain at a certain time and location to suddenly vanish, and then – a moment or so later – be replaced by an exact duplicate (this newly created brain does not vanish straight away, but endures). Needless to say, this sequence of events is far less probable than the simple creation of a single briefly-existing Boltzmann brain, but improbable is not the same as impossible. If the universe is infinite, it probably happens from time to time. For more on Boltzmann brains, see Sean Carroll, ‘Boltzmann's Anthropic Brain’, Discover, August 1, 2006.

⁴ Peacocke, The Mirror of the World, 69.

⁵ Dainton, Barry, The Phenomenal Self (Oxford: Oxford University Press, 2008)CrossRefGoogle Scholar.

⁶ The identity of an experiential capacity depends on three elements: the kind(s) of experience it produces when triggered, the conditions which trigger it, and the particular ground or base it has. If our own experiential capacities are properties of our brains, then their bases will, I take it, be neural systems of one kind or another. Whether or not the very rapid replacement of one's neurons with exact duplicates leads to a replacement of one's experiential capacities depends on the relationship between capacities (or dispositional properties) in general and their bases. If they are distinct, as some hold, then the original capacities may survive; if they are identical, as others hold, this won't be possible. But irrespective of which of these views is correct, the original subject's existence is not threatened provided there remains in existence an uninterrupted potential for fully continuous consciousness. (By way of analogy, think of the ‘third’ power-carrying rail of a transit system; there can be a continuous electrical potential between two locations x and y irrespective of whether the track in between consists of a single long piece of track, or several shorter laid end to end.) We will be looking at the relationship between capacities and their bases in more detail later on.

⁷ See my Stream of Consciousness (London: Routledge, 2000/2006)Google Scholar for further elaboration. I respond to some recent challenges to my account of diachronic unity in ‘Flow, Repetitions and Symmetries: Replies to Lee and Pelczar’, in Oaklander, N. (ed.) Debates in the Metaphysics of Time (London: Bloomsbury, 2014), 175–212.Google Scholar

⁸ Following Brentano (and ultimately Aristotle) many classical phenomenologists have endorsed the doctrine that consciousness necessarily involves some form of self-consciousness, recent advocates include Zahavi, D., Subjectivity and Selfhood (Cambridge: Cambridge University Press, 2005)CrossRefGoogle Scholar; Kriegel, U. Subjective Consciousness: A Self-Representational Theory (Oxford: Oxford University Press, 2009)CrossRefGoogle Scholar. For some skeptical doubts see Peacocke, Mirror of Nature, chapters 1–3, and Dainton, The Phenomenal Self , §2.4, §8.2.

⁹ Snowdon, Paul, ‘Persons, Animals and Ourselves’, in Gill, C. (ed.), The Person and the Human Mind (Cambridge: Cambridge University Press, 1990), 83–107 Google Scholar. Olson, Eric, The Human Animal (Oxford: Oxford University Press, 1997)Google Scholar; ‘An Argument for Animalism’, in Barresi, J. and Martin, R. (eds), Personal Identity (Oxford: Blackwell, 2003).Google Scholar

¹⁰ For a fuller exposition see The Phenomenal Self , §7.7; I also suggest a solution along these lines in my ‘Review of Eric Olson's The Human Animal , in Mind 107, (1998), 679–82Google Scholar. Galen Strawson adopts the same position in ‘The Self and the SESMET’, Journal of Consciousness Studies, 6:4, (1999), 99–135.Google Scholar

¹¹ Parfit, D., ‘We Are Not Human Beings’, Philosophy, 87:1 (2012), 5–28.CrossRefGoogle Scholar

¹² Olson, E., ‘The Nature of People’, in Luper, S. (ed.) The Cambridge Companion to Life and Death (Cambridge University Press 2014, 30–46)CrossRefGoogle Scholar; also his ‘On Parfit's View That We Are Not Human Beings’, in O'Hear, A. (ed.), Mind, Self and Person (Cambridge University Press, forthcoming)Google Scholar

¹³ ‘On Parfit's View That We Are Not Human Beings’, this volume.

¹⁴ In The Phenomenal Self (§8.1–8.3) I broach the question of how simple a subject of experience could possibly be. Could there be a subject with a mind so primitive that it possesses the capacity for just a single simple form of sensation? An issue anticipated by Hume: ‘We can conceive a thinking being to have either many or a few perceptions. Suppose the mind to be reduc'd even below the life of an oyster. Suppose it to have only one perception, as of thirst or hunger. Consider it in that situation’ ( A Treatise of Human Nature, (ed.) Selby-Bigge, Oxford: Clarendon Press, 1739/1978: 634Google Scholar) I argue that such minds may well be (logically) possible. If so, then the entirely isolated experiential capacities envisaged by Olson would constitute subjects of experience in their own right – or at least, they would if (as we are assuming) each produces an entirely separate stream of consciousness.

¹⁵ The issue of precisely how C-systems are related to brains is bound up with several very general metaphysical questions concerning (a) the relationship between objects and their properties, and (b) the nature of dispositional properties. To illustrate, for anyone inclined to identify dispositional properties with their physical bases it will be an option to maintain that in the human case, experiential capacities are identical with certain neural systems. On the resulting view, C-systems will almost certainly be proper parts of human brains, rather than properties of them. But irrespective of whether C-systems are parts of brains, or properties of them, it remains the case that they are less than entire brains, and so not identical with them.

¹⁶ Here I paraphrase a passage from Johnston's Surviving Death (New Jersey: Princeton University Press), 143Google Scholar.

¹⁷ Bayne, T., The Unity of Consciousness (Oxford: Oxford University Press, 2010), 288.CrossRefGoogle Scholar

¹⁸ Ibid., 288.

¹⁹ Such as his The Conscious Mind (Oxford: Oxford University Press, 1996).Google Scholar

²⁰ Strawson, G., Selves (Oxford: Oxford University Press, 2009)CrossRefGoogle Scholar. Strawson, notoriously, also holds that subjects only last a fraction of a second, as measured by clock time – but this is because he believes that our streams of consciousness are much briefer than most of us tend to think.

²¹ It is not completely uncontroversial. There are those – causal structuralists, for example – who hold that there can be capacities or dispositions which are not grounded in properties of a non-dispositional sort. Whether ‘bare dispositions’ are possible is an interesting question, but as far as I can see it is largely irrelevant to the present issue – for reasons which will emerge.

²² Armstrong, D., ‘Dispositions are Causes’, Analysis 30 (1969), 23–6CrossRefGoogle Scholar; Mellor, H., ‘The Semantics and Ontology of Dispositions’, Mind, 109 (2000), 757–780).CrossRefGoogle Scholar

²³ Prior, E.M., Pargetter, R., Jackson, F., ‘Three Theses about Dispositions’, American Philosophical Quartely, 19 (1982), 251–257.Google Scholar

²⁴ Heil, J., From an Ontological Point of View, (Oxford: Oxford University Press, 2003).CrossRefGoogle Scholar

²⁵ Mumford, S., Dispositions (Oxford: Oxford University Press, 1998).Google Scholar

²⁶ The complexity of some of the time travel scenarios devised by philosophers easily match the complexity of those devised by most Hollywood scriptwriters. In his ‘Who was Dr Who's Father?’ MacBeath defends the logical possibility of a case (devised by Jonathan Harrison) which implies ‘that a man can be his own father, that he can eat himself (that is, eat himself up), and that he can die before he is born.’ See MacBeath ‘Who Was Dr Who's Father?’, Synthese 51 (1982), 397.CrossRefGoogle Scholar

²⁷ Lewis, D., ‘The Paradoxes of Time Travel’, American Philosophical Quarterly 13 (1976), 145–152.Google Scholar

²⁸ Ibid., 70

²⁹ For two experiences E1 and E2 to be synchronically co-conscious it is not enough for them simply to be experienced simultaneously, i.e. be experienced at the same moment of ordinary objective time. E1 and E2 are only synchronically co-conscious if they are experienced together, i.e. unified in the same distinctive way as the current contents of your stream of consciousness.

³⁰ For comments on earlier drafts and discussion of the ideas therein, my thanks to Thomas Jacobi, Galen Strawson and Tom Winfield.