If you've spent any time with foreigners learning your language, you may have been in conversations like this:

Mei: I'm a bit confused ... what's the difference between "even though" and "although"?

Albert: Um, I think they're mostly equivalent, but "even though" is a bit more emphatic.

Barry: Are you sure ? I remember something about one being for positives, and the other for negatives. For example, let's see, these sentences sound a bit weird:"He refused to give me the slightest clue, although I begged on my knees", and "Although his car broke down on the first mile, he still won the rally".

People can't automatically state the rules underlying language, even though they follow them perfectly in their daily speech. I've been made especially aware of this when teaching French to Chinese students, where I had to frequently revise my explanation, or just say "sorry, I don't know what the rule is for this case, you'll just have to memorize it". You learn separately how to speak the language and how to apply the rules.

Morality is similar: we feel what's wrong and what's right, but may not be able to formulate the underlying rules. And when we do, we're likely to get it wrong the first time. For example you might say:

It has been suggested that animals have less subjective experience than people. For example, it would be possible to have an animal that counts as half a human for the purposes of morality.

But unlike grammar, people don't always agree on right and wrong : if Alfred unintentionally harms Barry, Barry is more likely to think that what Alfred did was morally wrong, even if both started off with similar moral intuitions. So if you come up with an explanation and insist it's the definition of morality, you can't be "proven wrong" nearly as easily as on grammar. You may even insist your explanation is true, and adjust your behavior accordingly, as some religious fanatics seem to do ("what is moral is what God said" being a quite common rule people come up with to explain morality).

So: beware of your own explanations. Morality is a complex topic, you're even more likely to shoot yourself in the foot than with grammar, and even less likely to realize that you're wrong.

(edit) Related posts by Eliezer: Fake Justification, Fake Selfishness, Fake Morality.