Among my many sins, I lecture on the law of contract and public law, and am finishing off a PhD on sovereignty. So, naturally, I think about the law a lot, in particular the philosophy of law, and one of the things I find interesting about older D&D is the "lawful" alignment - even though I know that what the designers were getting at and why they gave it that moniker is basically just all about aping Michael Moorcock.
Jurisprudence 101: almost all the philosophies of law there have ever been can essentially be broken down into two schools: positivism and natural law. (It's a little bit more complicated than that, though not much.)
Positivists, of whom the English philosopher of law HLA Hart is probably the most famous, approach law as something that is only 'posited' (hence the name) - i.e., that a society's legal system is not based on anything fundamental or moral about the universe or human nature, but merely on the rules that the society has agreed to operate on - and really those rules could be anything. Broadly, legal validity, for a classical positivist, depends not on the content of the rules or their merits, but on their sources: law is "normatively inert". (This does not mean that positivists ignore issues of morality and justice - simply that they do not see law as being connected with morality or justice except by coincidence.)
Natural law takes the opposite approach, assuming that law is inextricably linked to morality and justice, and natural law theorists assume that legal validity rests on that connection - a law is not valid unless it is based on some moral principle or other. This might be religious in nature, as Aquinas would have argued, or it might be based on a concept of fundamental 'human goods' a la John Finnis; either way, a law is not a law unless it has normative content. (Ronald Dworkin, perhaps the only legal philosopher non-lawyers might have a cat's chance in hell of knowing about, was basically a believer in natural law despite his protestations: his view that judges interpret the law based on "principles", and that every case has a "right answer", is classic natural law.)
To caricature things somewhat, to the positivist, the problems with natural law are twofold: it assumes that law is subjective and hence arbitrary (what are "morals", "human goods", and "principles", and who gets to decide? - one of the key criticism's of Dworkin's work is that, by amazing coincidence, it turns out that "principles" are basically liberal/social democratic in nature, which is Dworkin's personal political persuasion), and it suggests that there is no such thing as value pluralism (because ultimately there are "right answers"). To the natural lawyer, the problems with positivism are also twofold: it does not really reflect reality to assume that there is no necessary connection between law and morality/justice, because in practice everybody behaves as if the two are linked; and without moral content law becomes oppressive (a decree to kill all Jews is still a law unless you assume that "law" implies a certain moral foundation).
This is all well and good, of course, but how does it relate to D&D?
It is my contention (not that I've ever contended it in public) that there are two corresponding approaches to the 'lawful' alignment in Classic D&D. On the one hand, it is possible to take the natural law perspective, which ultimately suggests that 'lawful' is synonymous with 'good'. A lawful character is moral (in whatever sense), and believes in some concept of truth, justice and righteousness which is the foundation on which society's rules rest, and behaves accordingly.
On the other hand, however, it is perfectly feasible to construe a lawful character as being a positivist, i.e., a believer in a certain set of laws which do not necessarily have a moral or ethical foundation (although which might do, of course), but which are simply set by the society in which he or she grew up. This opens the door to a wide variety of different, and weird, notions of what 'laws' and 'rules' are. Maybe your lawful character holds that to hold property is unlawful? That there is no such thing as 'theft'? That working on a Sunday is against the rules? That a lie is not a lie if you are standing on one leg while saying it?