What do I mean by "morality isn't logical"? I mean in the same sense that mathematics is logical but literary criticism isn't: the "reasoning" we use to think about morality doesn't resemble logical reasoning. All systems of logic, that I'm aware of, have a concept of proof and a method of verifying with high degree of certainty whether an argument constitutes a proof. As long as the logic is consistent (and we have good reason to think that many of them are), once we verify a proof we can accept its conclusion without worrying that there may be another proof that makes the opposite conclusion. With morality though, we have no such method, and people all the time make moral arguments that can be reversed or called into question by other moral arguments. (Edit: For an example of this, see these posts.)
Without being a system of logic, moral philosophical reasoning likely (or at least plausibly) doesn't have any of the nice properties that a well-constructed system of logic would have, for example, consistency, validity, soundness, or even the more basic property that considering arguments in a different order, or in a different mood, won't cause a person to accept an entirely different set of conclusions. For all we know, somebody trying to reason about a moral concept like "fairness" may just be taking a random walk as they move from one conclusion to another based on moral arguments they encounter or think up.
In a recent post, Eliezer said "morality is logic", by which he seems to mean... well, I'm still not exactly sure what, but one interpretation is that a person's cognition about morality can be described as an algorithm, and that algorithm can be studied using logical reasoning. (Which of course is true, but in that sense both math and literary criticism as well as every other subject of human study would be logic.) In any case, I don't think Eliezer is explicitly claiming that an algorithm-for-thinking-about-morality constitutes an algorithm-for-doing-logic, but I worry that the characterization of "morality is logic" may cause some connotations of "logic" to be inappropriately sneaked into "morality". For example Eliezer seems to (at least at one point) assume that considering moral arguments in a different order won't cause a human to accept an entirely different set of conclusions, and maybe this is why. To fight this potential sneaking of connotations, I suggest that when you see the phrase "morality is logic", remind yourself that morality isn't logical.
There's a pseudo-theorem in math that is sometimes given to 1st year graduate students (at least in my case, 35 years ago), which is that
All natural numbers are interesting.
Natural numbers consist of {1, 2, 3, ...} -- actually a recent hot topic of conversation on LW ("natural numbers" is sometimes defined to include 0, but everything that follows will work either way).
The "proof" used the principle of mathematical induction (one version of which is):
If P(n) is true for n=1, and the assertion "m is the smallest integer such that !P(m)" leads to a contradiction, then P(n) is true for all natural numbers.
and also uses the fact (from the Peano construction of the natural numbers?) that every non-empty subset of natural numbers has a smallest element.
PROOF:
1 is interesting.
Suppose theorem is false. Then some number m is the smallest uninteresting number. But then wouldn't that be interesting?
Contradiction. QED.
The illustrates a pitfall of mixing (qualities that don't really belong in a mathematical statement) with (rigorous logic), and in general, if you take a quality that is not rigorously defined, and apply a sufficiently long train of logic to it, you are liable to "prove" nonsense.
(Note: the logic just applied is equivalent to P(1) => P(2) => P(3), ...which is infinite and hence long enough.)
It is my impression that certain contested (though "proven") assertions about economics suffer from this problem, and it's hard, for me at least, to think of a moral proposition that wouldn't risk this sort of pitfall.
http://en.wikipedia.org/wiki/Interesting_number_paradox and http://en.wikipedia.org/wiki/Berry_paradox