I am glad you think the argument has some merit.

I mean by "X is an essential property of Y" metaphysical essence - so, something that makes it the kind of object it is. I would take shape and size to be essential properties of physical objects, whereas 'colour' does not seem to be (though that is just to illustrate what I mean, but it would not affect my case if colour was an essential attribute of physical things).

I agree that if physical things are essentially conscious then that would stop the argument. But consciousness does not seem to be an essential feature of physical things. Those who believe us to be physical things do not - I think - typically hold that we are essentially conscious. Consciousness would then have to be held to be a feature of all physical things (and by extension, they would have to hold that all physical things are equally intrinsically morally valuable - which seems false).

My premise that consciousness is not an essential feature of physical things is not equivalent to denying that any physical things are conscious, for it is consistent with consciousness not being an essential feature of such things that nevertheless, some have that feature (just as, by analogy, if colour is not an essential feature of physical things, that does not prevent physical things from having colour). However, if the argument as a whole is sound, then I think it would establish that no physical thing is conscious. For if we are morally valuable because we are things of a sort that are conscious, then that would be an essential property of the kinds of thing we are, and as that is not an essential feature of physical things, the sorts of thing that have consciousness would have been demonstrated to be non-physical.

No, I wouldn't say that the attitude is intrinsic to the thing. Rather, something essential to the thing is what is responsible for my valuing attitude.

To use my valuing of something as an example, if I value something intrinsically, then I would be valuing it due to something essential to it, whereas if I value something extrinsically, then I would be valuing it due to something non-essential about it. Were I to say that I find something intrinsically valuable, then, I would be saying that I value it due to some of its essential properties, rather than saying that my valuing of it is an essential property of that thing.

Applied to moral value, for something to be intrinsically morally valuable is for it - the thing - to have moral value due to some its essential properties. I think that's correct anyway.

Thank you for your reply.

That something is morally valuable would be a property of that thing. But it would be a supervenient property, meaning that it is resultant from some of the thing's other properties. The difference, I take it, between something being 'intrinsically' morally valuable and 'extrinsically' morally valuable is that in the former case the moral value is supervening on essential properties of the thing, whereas in the latter case it is not.

So all moral value - whether possessed intrinsically or extrinsically - supervenes on something's other features. But intrinsic value supervenes on something's essential features. I think that's right, anyway.