I'm not so much talking about the legal definitions, as about the basic intuitions that form the framework for the moral reasoning that goes into determining behaviour and then the formal laws and systems that govern them.
It's one of the priors that someone with a non magical upbringing may never consider, that the basic foundation of moral reasoning is different for pureblood wizards.
That other sapient beings have weight as moral actors is pretty basic, and if pure bloods were to instead use a different intuition as the starting point for moral construction, then Harry has a very substantial amount more work to do.
n.b., I have to admit that I was rather disappointed by their being a physical basis for magical ability that proved Harry was right and the pureblood faction wrong. I think it would make a far more interesting setting if the pure bloods were actually factually correct but still morally wrong. Just as interesting would be there being no physical basis for magical ability, and it simply being an example of large scale magic such as the taboo or the cure on the DADA job, the equivalent of a curse or blessing on a family line.
Sorry, I didn't mean to make it about law. I just happen to find that particular definition pretty intuitively appealing; that the definition was canon magical law was a minor side point of only marginal relevance.
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.