I find Frank Jackson’s arguments against non-naturalist realism in his From Metaphysics to Ethics fascinating. So I think that, over a longer period of time, I would like to post a couple of things about them. Today, I am puzzled about the one that has received most attention. I wonder whether this argument is available for Jackson himself.
Very roughly, as I understand it, this argument begins from the premise that, given the truth-aptness of moral claims and global moral superveniece, for every moral property, there must be a naturalist property that is necessarily co-extensive with it. This property can perhaps, at worst, be a very disjunctive property consisting of full descriptions of every words of which the given moral term can be truthfully used.
The second premise of the argument is that no necessarily co-extensive properties can be distinct ones. Necessary co-extensiveness must imply identity. From these two premises the wanted conclusion follows; truth-aptness and supervenience imply that moral properties cannot be non-natural properties. Rather, they must be natural properties.
The latter, essential second-premise is becoming much debated. Russ Shafer-Landau (in his Moral Realism) and Brad Majors (in his “Moral Discourse and Descriptive Properties” in Philosophical Quarterly, 2005) present a variety of counter-examples to the principle according to which necessary co-extensiveness guarantees identity. Many of these examples are discussed and undermined in Jackson’s original presentation and in Bart Streumer’s forthcoming “Are There Irreducibly Normative Properties?” (in Australasian Journal of Philosophy). It would be nice to talk about these cases too but I’ll leave it to some other time.
What puzzles me most about the argument is that Jackson himself, as far as I can tell, offers a perfectly good counter-example to his own principle. This seems to happen in a paper he wrote with Elizabeth Prior and Robert Pargetter called “Three Theses about Dispositions” (American Phil. Quarterly 1982) and also in the Chapter 4 about colours in From Metaphysics to Ethics.
In these works, Jackson argues that dispositional properties must have categorical properties as their causal bases. That is, something that is fragile must be fragile because it has some molecular structure that causes it to shatter when dropped. Now, Jackson accepts that fragility can be multiply realisable. A fragile object can have the molecular structure alpha, or beta, and so on so long as these structures are such that the object shatters when dropped.
Now, it seems that there must be a disjunctive property consisting of all the causal bases for fragility (Jackson accepts that a similar property for the causal bases for the property of looking yellow on page 106 of fMtE where he wants to identify the colour yellow with this disjunctive property). It also looks like this disjunctive property must be necessarily coextensive with fragility.
But, now, Jackson himself argues that this disjunctive property is not identical with the dispositional property. He argues that, given that the disjunctive causal bases property of the dispositional property is not excessively disjunctive, it has causal powers – it causes the causal manifestations of the relevant disposition. However, dispositional properties are, he argues, causally impotent. Fragility does not cause the glass to break, its molecular structure does and the forces upon it.
This difference between the causal powers of dispositional properties and the disjunctive properties that are the causal bases of such properties seem to imply by the law of indiscernibility of identicals that these two properties are distinct – as Jackson seems to explicitly accept. But, these properties are necessarily coextensive. Thus, there must be distinct necessarily coextensive properties. So, it looks to me that either Jackson must give up his view about dispositions or the crucial premise in his argument against non-naturalism. But, I do trust Jackson more than myself so I must have missed something.