First, a question about whether Aristotle can apply to us today.
Aristotle explicitly says at 1326a that the statesperson ought to consider the size of the ideal state, and he clearly says that a very populous state cannot be well-governed. He argues as follows: law is order, and good law is good order. A very great multitude cannot be orderly; to introduce order into the unlimited is the work of divine power. The conclusion must be that there is a limit to the size of a well-governed state. The minimum size is the number of people required to be sufficient for well-being: in other words, sufficient material goods must be produced so that there are enough for the state to perpetuate itself.
It seems clear that he would reject even a state the size of Liechtenstein, nevermind the USA.
The first of two reasons he gives for limit to size is that there cannot be a general or herald for a multitude above a given size. In other words, the technology of communication (the human voice, letters, messengers) made it impossible to efficiently run a larger multitude. The second reason he gives is that "if the citizens of a state are to judge and to distribute offices according to merit, then they must know each other's characters; where they do not possess this knowledge, both the election to offices and the decision of lawsuits will go wrong." The technology of communication made larger electorates problematic, for how could they know the candidates?
Now I ask, would modern technology persuade Aristotle that the
ideal state can be much larger today than it could be in his day?
Modern communication across many media may make the Aristotelian
revise Aristotle. But rhetorical analysis may require rejection of
certain media (the medium is the message: TV, and more so X, etc.
in particular may be problematic, because they convey images and
very brief impressions much better than arguments?).
Aristotle thought that different subjects are amenable to different degrees of exactitude. Politics is not amenable to a high degree of exactitude.
A crucial part of Aristotle's method is found in the following:
Aristotle's predecessor Plato discusses what would make a city
happy,: while it is hard to determine how it works, it is clear
that the happiness of individuals is not absolutely necessary for
a happy state. Think of a large crowd of sailors: no individual
sailor need be large for the crowd itself to be large. Aristotle,
however, rejects that idea.
He holds that humans are
political animals, in other words, they must be involved in a society in
order to be fully human. Their nature is social, and one
of the most important social orders is political. Other social
orders are familial, and whatever Aristotle would call the order
that is achieved by slaves and other non-citizens.
Our social nature makes it the case that we cannot achieve
well-being without being social. If we are citizens, we can
achieve well-being to a greater degree than if we were
non-citizens. The happiness of a polis is the happiness of its
citizens. In other words, happiness
belongs to a state like anger belongs to a crowd, not
like largeness: a crowd of angry sailors is an angry crowd of
sailors, and you cannot have an angry crowd of sailors unless you
have individual angry sailors.
The happy life is the life of virtue. The life of virtue is the one that allows the fullest development of our natures: the one that allows us to be excellent. When a person is completely virtuous, they must, of necessity, be virtuous towards others, both because it is virtuous to be virtuous to others, but also because our social nature cannot be excellently expressed unless it unfolds in the company of others.
It is not simply a matter of the fact that we need others to survive: it's not like breathing or digesting or food supply. We also need others in order to fully achieve excellence. Certain functions can only be fulfilled in a society. Our individual goals are not the only goals that we try to fulfill when we develop excellence. There are goals larger than our individual ones (e.g. an educational system that will produce further virtuous people: we need them to be virtuous as well).
How virtues overlap and are all one: If I have more food than I need and refrain from eating more than is good for me because that will be good for me, I am temperate. On the other hand, when I have more than I need, and I refrain from eating beyond satiation in order to give some to others who need it because that is good, I am JUST and temperate. In other words, justice is a given virtue excercised in a social way, namely for others.
Aristotle thought there was a fact of the matter about justice: it is not purely conventional. How did he address those who claim that it is purely conventional?
"It seems to some that everything is of this sort [just only by convention] because what is by nature is supposed to be unchanging and have the same force everywhere, like fire which burns both here and in Persia, and they see what is just changing. In a way this is right, but in a way it is not. Perhaps among the gods what is by nature does not change, but among us it is possible for something both to be by nature and to be entirely changeable: there is nevertheless a difference between what is by nature and what is not by nature. It is clear, among the things which can be otherwise which are by nature and which are not but are conventional and by agreement, despite the fact that both are similarly changeable. The distinction is the same in other cases. The right hand is by nature stronger, although everyone could become ambidextrous. Things which are just by agreement and in accordance with advantage are like measures; measures for wine and corn are not the same everywhere but are larger in wholesale markets and smaller in retail. Similarly, just things which are not natural but of human devising are not the same everywhere, otherwise constitutions would be the same and they are not. Nevertheless, only one is the best everywhere in accordance with nature." EN 1134b24-1135a5.
It is best to remember that some regularities in nature are exceptionless (gravity), and others are for the most part true (that humans are bipedal).
For some things, it does not matter which way they be, but it
very much matters that they be. For example, how you honor your
parents is a matter of convention, but that you do is natural.
Whether we use inches, meters, or whatever is a bit less important
than that we have standardized measures.
Any enactment intended to apply to only one situation is not a law, it is a decree, and it is always only by convention. The narrowness of the enactment is the key to understanding this: specific situations do not repeat themselves. Natural justice operates on more general level than specifics. I.e. if it is naturally just to stand by one's fellow in battle, that does not in any way make it just in every situation to do so: natural justice does not apply directly to specific situations, but must be put through a filter of "equity" (1137b11ff.) If, however, per impossibile, the particular specifics of a given case were to recur, the same thing would be just in both cases (but it could be a morally neutral matter which simply needs A decision, not THIS decision).
That said, it is nonetheless clear that Aristotle thinks that most variation in beliefs about justice arises not from differences of specifics or morally neutral matters: variation arises from errors of judgement about natural justice.
Although many laws may be merely conventionally just, it seems that as a meta-law principle, Aristotle would hold that it is naturally just that one obey such laws qua a system of laws if not qua this system of laws.