Classical texts are often cited differently than modern texts. This is usually due to one of two reasons.

First, some classical texts were organized in such a way by their authors (e.g., by "book," chapter, section or paragraph numbers, etc.), and over time this has become the standard way of referencing such works. For example, Thomas Hobbes divided his Leviathan (1651) into four main parts, each of which has its own set of chapters. These chapters, in turn, are each made up of their own set of numbered paragraphs. Thus, it has become standard in papers which repeatedly cite this text to reference passages by part, chapter and paragraph number. For example:

Subsequent citations would be abbreviated to read, for example:

The second reason classical texts are cited differently is because early editors or translators of these works imposed on them an organizational structure that differed from that of the author's. These later organizing principles were then adopted by subsequent editors until referencing these works by these later principles became standard. This is especially true of ancient classical texts. For example, it is now standard to cite Plato by what are called the "Stephanus" numbers which run down the margin of a good edition of Plato's works. For example, citations from Plato's dialogue Euthyphro would look like this:

Subsequent citations would simply read:

It's the same thing with works by Aristotle, which have been organized by "book," section, what are called "Bekker numbers," and line numbers. However, as a general rule of thumb, one should only give as many numbers as are necessary to allow someone to locate the passage cited with the greatest degree of precision. Thus, it is customary to cite Aristotle by work, and Bekker and line numbers. For example:

Subsequent citations would read:

So why bother with these odd, esoteric citation systems? Why not just use the MLA or Chicago styles, like we do for more modern texts? The answer is simple: these classical texts have over time become such a part of our common culture through so many editions produced by so many editors and translators over the centuries that the only way to make accurate references across all of these different editions is by referring to the organizing principles they have come to have in common. Try to imagine citing the Bible without referring to chapter and verse, and you'll start to get what I'm talking about. The only time you should use a modern citation style on a classical work is if you have a substandard edition that is not arranged by the work's traditional standard of organization.

A parting word of caution: Don't mistake the editor or translator for the author! For example, G.M.A. Grube is a famous translator of Plato's works--but he is not the author, Plato. Similarly, Edwin Curley is an editor of Hobbes's Leviathan, but he is not Hobbes. The only time I should see an editor's or translator's name is on the works cited page, with the rest of the publication information of the text you are citing.