- The interpretation of quantum theory1 and the relation of mind to matter seem very disparate topics. But it is not foolhardy to link them, for two reasons. The first reason is very general. Any description of the world that someone advocates as being complete, or as an outline of such a complete description, must 'close the circle': it must include an account of how we come by that description. In particular, any physical theory that claims such completeness must account for our experience as observers. This requirement has of course been accepted since the rise of modem physics. Galileo and his successors, such as Hobbes and Descartes, sketched physicalist accounts of, for example, our experience of colours: accounts which have become vastly more detailed since the mid-nineteenth century, thanks to such figures as Helmholtz. The second reason is specific to quantum theory2 (QT). Despite QT's supreme empirical success, it faces an interpretative problem, which suggests that it could never account for our experience as observers. The problem arises from a central feature of its success in describing the microrealm of electrons, atoms etc.: namely, its denial that these objects always have definite values for quantities like position, momentum and energy. In a nutshell, the problem is that QT apparently implies that this indefiniteness should also be endemic in the macrorealm, in the familiar realm of tables and chairs. But tables and chairs surely have definite positions etc.; at least, we experience them as doing so. So, if QT is to account for our experience, it must either secure such definiteness, or at least explain the appearance of it.
- This problem is called the 'measurement problem', mainly because the argument that QT implies an indefinite macrorealm is clearest for a measurement situation. Thus, if you use QT to analyse a measurement of , say, the momentum of an electron, which QT says has no definite momentum, you find that according to QT, the indefiniteness of the electron's momentum is transmitted to the apparatus’ pointer – so that it has no definite position.
- There are several general strategies for solving the measurement problem, each strategy comprising various views. As we shall see, these views differ widely, not just in their technical details, but in their basic world-pictures: so there is honest work for a metaphysician to do, articulating and comparing these world-pictures. For the moment, suffice it to say that, for our topic of QT and mind, the main contrast is between: (i) those views that claim to secure a definite macrorealm, and so to explain why the macrorealm is as it appears to be; and (ii) those, perhaps more radical, views that allow an indefinite macrorealm and claim only to secure definite appearances (thus denying that it is as it appears to be). I shall concentrate on the second group, especially on the Many Worlds, and the Many Minds, views inspired by the work of Everett.
- The plan of the paper is as follows. Section I gives more details about the measurement problem. Section II discusses the role of the environment and a phenomenon called 'decoherence'. Section III surveys strategies for solving the problem. Section IV looks briefly at some views in group (i) above, that propose to revise QT's basic laws; namely, the dynamical reduction proposals of Ghirardi, Pearle and others, and the proposals of Penrose and Stapp. These proposals are relevant to our topic of QT and mind, since they each bear on the relation of mind and brain (albeit in very different ways).
- The rest of the paper focuses on views that allow an indefinite macrorealm. Sections V to VII discuss the Many Worlds view, without regard to minds. Instead, they relate the view to such standard philosophical topics as possible worlds, indexicality and transtemporal identity. Sections VIII to X discuss the Many Minds view. Admittedly, the distinction between Many Worlds and Many Minds is delicate; in particular, it seems likely that Everett intended to express a Many Minds view – as several commentators maintain. But the distinction will be convenient for exposition: in Sections V to VII, it will be easiest to connect the Many Worlds view with the standard philosophical topics, if we set aside issues about the mind. When I turn to Many Minds, I will again need to specialize, because of lack of space. I will focus on the views of Albert & Loewer, and Lockwood. But I will end by briefly praising Donald's alternative version (Section X).
Symposium on "Quantum Theory3 and the Mind". See "Fleming (Gordon N.) - Quantum Theory and the Mind" for a reply.
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