DrSinger: Summary

Steel-manning NTT

Before proceeding we will rephrase the argument in the following way, to capture its intended meaning

(P1) All sentient humans have moral value

(P2) There is no trait absent in sentient non-human animals which is such that, if the trait were absent in a counterpart to a sentient human, then they would be not have moral value.

Therefore, (C) All sentient non-human animals have moral value

Here we have introduced the notion of the counterpart to a sentient human, to mean a replica of a sentient human with the trait removed, that does not have to remain a sentient human.

[Margaret I'm basically fine with this, and it was largely my intention. But I still think that the argument as originally presented in English did require the counterparts to still be us / possess our essential properties; this version has some rational force and is not question-begging. It should be addressed somewhere]

This is necessary because if we were to interpret P2 as;

There is no trait absent in sentient non-human animals which is such that, if the trait were absent sentient humans, then they would be not have moral value

then P2 would be rendered vacuously true, since P1 says sentient humans always have moral value.

[Margaret No; P1 is just saying that sentient humans as they actually are have moral value. It isn't committing at all to whether they would have moral value if certain changes were made to them - including only identity preserving changes or changes in accidental properties]

Summary of Issues

As we will see below, the reason NTT fails is that it is possible to show a counterexample in which, sentient humans have moral value, there exists a sentient nonhuman animal that lacks moral value, and that for all traits, human counterparts possess moral value with the trait removed.

[Margaret: I still think that we should at some point say that the natural interpretation of this counterexample is value narcissim, and it comes up in the debates, and it works against the argument as interpreted in ordinary English]

Ask Yourself has argued that if one were to lose all of the traits that distinguish one from a non-human animal, then the resulting entity would be identical to that non-human animal, so P1 and P2 so interpreted [logically] entail C. The problem with this claim is that it presupposes something like the identity of indiscernibles: that if two entities have all of the same properties, then they must be the self-same object. This is a substantive metaphysical thesis, to which certain philosophers have objected (for discussion see [1]), so it must be added as an additional premise for the argument to be logically valid.

But as we will see below, the much greater problem with the argument so interpreted is that it has essentially no rational force. P2 so interpreted is essentially just asserting that all sentient beings, human and non-human, have moral value. The argument thus offers little if any reason to change the mind of someone who does not already find this view plausible, and the defense of its premises offers no guidance on how to persuade such an individual.