So morality has a lot to do with logic — indeed I have argued that moral reasoning is a type of applied logical reasoning — but it is not logic “all the way down,” it is anchored by certain contingent facts about humanity, bonoboness and so forth.
But, despite Yudkowsky’s confident claim, morality isn’t a matter of logic “all the way down,” because it has to start with some axioms, some brute facts about the type of organisms that engage in moral reasoning to begin with. Those facts don’t come from physics (though, like everything else, they better be compatible with all the laws of physics), they come from biology. A reasonable theory of ethics, then, can emerge only from a combination of biology (by which I mean not just evolutionary biology, but also cultural evolution) and logic.
To clear everything up:
The first in the op argues morality as logic because it isn't logic "all the way down". Yuan is saying that all the way down doesn't matter because it works within its own context and that, that is all that matters. Obviously this is wrong; creating a context just so you can work within it to prove your point is known as a strawman. The logic of the context is just as important as the logic it contains.
....I don't see why. We can tell a story about how our desires came into being, through psychology and the like, but when we ask 'what do we do right now?' the calculation doesn't need to refer to those facts. It oughtn't - they aren't related to the moral calculation.
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Pigliucci:
http://rationallyspeaking.blogspot.de/2013/01/lesswrong-on-morality-and-logic.html