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Frank Jackson, From Metaphysics to Ethics (Chapter 5 The Location Problem for Ethics: Moral Properties and Moral Content)

* Jackson understands 'cognitivism' as a semantic thesis: ethical sentences are truth-apt, where to be truth-apt is to be semantically able to have a truth-value (113). 

[A Response to Jackson's Analytic Discriptivism: the Possibility of Logically Equivalent Predicates Picking Out Distinct Properties] 

Being an equilateral (three sides are the same length) triangle 

Being an equiangular (all three angles are equal) traiangle 

They are logically equivalent properties. Then, do they represent different properties? 

One argument: we can understand that a triangle is equilateral, but fail to see that it is also equiangular. So, the argument goes, they represent distinct properties. 

Jackson's response: on the conception of property accroding to which a property is a way things might be which is an aspect of the world, there is only one property represented by both the concept of being equilateral and being equiangular. 

Another argument: we can create a machine which detect a triangle which is equilateral, but not equiangular. This case shows that the former property has a unique causal function which is not shared by the latter. 

Jackson's response: this does not show whether being equilateral and being equiangular are distinct. We think that this example shows that only the former plays the causal role, but this judgement comes from the doctrine that sides are distinct from angles. This doctrine is the one which is in question, so the example does not support the conclusion the argument is supposed to provide. 

Jackson also provides some considerations which positively support the thesis that logically equivalent properties are not distinct. 

(1) Knowing the complete stroy of how and when the language user produces the word 'right' can be given descriptively, it is difficult to justify the idea that the property picked up by 'right' is distinct from the relevant discriptive properties. 

(2) 'Secondly, it is hard to see how the further properties could be of any ethical
significance. Are we supposed to take seriously someone who says, ‘I see
that this action will kill many and save no‐one, but that is not enough to
justify my not doing it; what really matters is that the action has an extra
property that only ethical terms are suited to pick out’? In short, the extra
properties would ethical ‘idlers’'. (127)

→ This consideration is supposed to be a one which supports the thesis that logically equivalent properties are identical. Here, Jackson seems to be saying that only descriptive properties have ethical significance which makes the action in question right or wrong while distinct properties do not have such significance. 

(3) It is hard to provide a principle which tells us when certain twining properties (such as of ethical) are distinct from the relevant descriptive properties. 

 * Jackson says certain metaphysical views are necessary for the supervenience of the psychological on the physical. On the other hand, we do not need to hold any metaphysical view on ethical properties in order appreciate the supervenience of the ethical on the descriptive (128-9).