The Principle of Naturalistic Closure and Naughty Metaphysics

A couple of people asked me what I meant by "polemics", in the previous post. I was referring to a recent book, Ladyman & Ross et al. 2007, Every Thing Must Go: Metaphysics Naturalized, whose Preface begins:

This is a polemical book. One of its main contentions is that contemporary analytic metaphysics, a professional activity engaged in by some extremely intelligent and morally serious people, fails to qualify as part of the enlightened pursuit of objective truth, and should be discontinued.

As a kind of criterion for identifying the allegedly naughty metaphysics which "should be discontinued", Ladyman & Ross et al. introduce to The Principle of Naturalistic Closure, denoted PNC:

The Principle of Naturalistic Closure
Any new metaphysical claim that is to be taken seriously at time t should be motivated by, and only by, the service it would perform, if true, in showing how two or more specific scientific hypotheses, at least one of which is drawn from fundamental physics, jointly explain more than the sum of what is explained by the two hypotheses taken separately, where this is interpreted by reference to the following terminological stipulations:

Stipulation: A ‘scientific hypothesis’ is understood as an hypothesis that is taken seriously by institutionally bona fide science at t.

Stipulation: A ‘specific scientific hypothesis’ is one that has been directly investigated and confirmed by institutionally bona fide scientific activity prior to t or is one that might be investigated at or after t, in the absence of constraints resulting from engineering, physiological, or economic restrictions or their combination, as the primary object of attempted verification, falsification, or quantitative refinement, where this activity is part of an objective research project fundable by a bona fide scientific research funding body.

Stipulation: An ‘objective research project’ has the primary purpose of establishing objective facts about nature that would, if accepted on the basis of the project, be expected to continue to be accepted by inquirers aiming to maximize their stock of true beliefs, notwithstanding shifts in the inquirers’ practical, commercial, or ideological preferences. (Ladyman & Ross (et al) 2007, Every Thing Must Go, pp. 37-8.)

It seems to me that there are problems with PNC, at least as stated, and the crucial phrase is

"metaphysical claim"

A definition of this is repudiated. But, for example, are claims like,

i) spacetime contains closed timelike curves
ii) spacetime permits hypercomputation

to be counted as metaphysical? The question is not whether they are scientific or not (they are scientific, in some reasonable sense). The question is whether they are metaphysical or not.

One might think, with some logical empiricists, that no scientific claim is a metaphysical claim. But then that just rules out naturalized metaphysics.

If, on the other hand (as seems reasonable to me), there is non-trivial overlap between metaphysical claims and scientific claims, then it would be useful to obtain some guidance as to how to identify the claims in the overlap, even if the guidance is a bit vague.

There presumably are also metaphysical claims which are somehow non-scientific (perhaps the existence of possible worlds, or transfinite cardinals, the existence of angels dancing on pins, etc.), but this raises demarcation questions about what a scientific claim is. If one adopts a sufficiently holistic epistemology, then - at least in principle, through pragmatic, coherence and simplicity considerations, but still conditioned by sensory experience - such questions become amenable to rational acceptance/rejection. (Quine's view.)