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 a state the size of Liechtenstein, and more so 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 again makes larger electorates problematic, for how can 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?
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: "The investigation of truth is in one way hard, in another easy. Evidence of this is that while no one is able to attain the truth adequately, no one fails entirely either, but rather each says something about the nature of things, and although individually they contribute little or nothing to the truth, something sizeable comes from all of them taken together." (Metaphysics 993a30-b4)
In Plato, we discussed what would make a city happy, and we found that it is hard to determine how it works, but it was clear that the happiness of individuals was not absolutely necessary for a happy state. Aristotle rejects that idea roundly. 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: 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. " It is impossible for the city to be happy as a whole unless all, or most, or some, of the parts are happy. Happiness is not like evenness, which can belong to the whole without belonging to either (or any) of the parts; this is impossible in the case of happiness." Politics 1264b17).
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 fact that we need others to survive. 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).
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.
Any enactment intended to apply to only one situation is 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 only through "equity" (1137b11ff.) If, however, the particular specifics of a given case were to recur, the same thing would be just in both cases (unless it is 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: it arises from errors of judgement about natural justice.
Although many laws may be merely conventionally just, it seems that 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.
"Just as a sailor is one member of a community, so too do we
say is the citizen. Insofar as sailors differ in capacity (for one is a
rower, one a pilot, one a lookout, another has yet another name of this
kind), it is clear that the most accurate account of the virtue of each
will be peculiar to each. Yet there is also a common account which fits
all. For the preservation of the ship is the function of all of them;
each of the sailors aims at this. The same holds for citizens. Although
they are dissimilar, the preservation of the community is their
function, and the constitution is the community. For this reason the
virtue of a citizen is necessarily relative to the constitution. Since
there are many forms of constitution, it is clear that there cannot be
a single and complete virtue of the excellent citizen, but we do say
that a man is good by having one virtue, which is complete. It is clear
then that it is possible for someone to be an excellent citizen without
possessing the virtue in accordance with which he would be an excellent
man." Politics 1276b20-35
The best polity