Many minds are no worse than one. (quantum theory)(response to M. Lockwood, in this issue, p. 159)

1 Introduction

The Everett-style interpretation of quantum mechanics developed by Michael Lockwood (the many minds view henceforth) is certainly very weird. Nevertheless, it may be true. Its strength is that it promises to save the appearances without wave function collapses or non-local interactions. Its weakness is that it seems to deny some obvious common-sense truths.

It is worth noting, however, that the common-sense assumptions denied by the many minds view involve either consciousness or objective probability. And the striking thing about these two notions is that neither is well integrated into the rest of our world view. How do conscious facts relate to non-conscious facts? And how do probabilistic facts relate to non-probabilistic facts? These are two of the most baffling questions in philosophy, and nobody has any good answers. In practice we use certain operational links to tie conscious and probabilistic facts to non-conscious and non-probabilistic facts respectively. Yet we lack any cogent philosophical justification of these links.

In this note I want to suggest that our understanding of consciousness and objective probability would be no less satisfactory on the many minds view than on a more conventional view of reality. Indeed the situation would be quite comparable. The many minds view would preserve the 'operational links' which connect conscious and probabilistic facts to the rest of the world, and would therefore leave us with the problem of explaining these links. Moreover, the many minds view would not offer any obvious solution to this problem. But then, as I said, neither does our conventional view of the world.

True, the many minds view would force us to change some familiar common-sense assumptions about consciousness and probability (indeed rather more, I shall suggest, than Michael Lockwood recognizes). But this is arguably a cost worth paying. For it seems to me that the threatened assumptions are backed by nothing except familiarity. As we shall see, they do not matter to our operational use of the notions of consciousness or probability. And they have no theoretical backing, since we have no good theories of consciousness or probability to start with.

So I want to suggest that the common-sense assumptions rejected by the many minds view are unmotivated, free-floating 'danglers', which are therefore up for grabs. Some people will no doubt take the weirdness of denying these assumptions as itself an argument against doing so. But mere unfamiliarity seems a poor argument against the only view that promises to explain the appearances without positing ad hoc or physically impossible mechanisms.

2 Consciousness

According to the many minds view as developed by Lockwood, if you observe a cat in a superposition of 'live' and 'dead', say, then your brain will itself become a superposition of 'registering live' and 'registering dead'; there is no physical point at which the wave function 'collapses'. Moreover, Lockwood's version of the many minds view holds that conscious mental events supervene on physical events. So at the conscious level too you will register 'live' and also register 'dead'.

On the face of it, this seems to contradict our experience. Surely we either see a live cat or a dead cat, but not both. However, we need here to consider what it would be like to have a superposed brain. A first thought might be that it would be like seeing a …