Plato is perhaps the most approachable of the major philosophers. His work is largely presented in short dialogues. Their brevity allow them to be read in a single sitting, and their characterizations, humor, and stories engage even the reader new to philosophy.Given this, it may surprise those unfamiliar with Plato to learn that the interpretation of him has always been the subject of hot dispute - perhaps only Nietzsche among philosophers has inspired more controversy.
Why is this? Why is Plato so easy to read and yet so difficult?
Five problems are worth calling out:
(1) Dramatic presentation: All of Plato's published works are presented as dialogues between characters - Plato himself is never a character. Thus, any interpretation must have some mapping (implicit or explicit) between the characters' views and Plato's views, as well as how the dramatic structure (setting, characters, story) as a whole presents Plato's views.
(2) Irony: The main speaker in most of Plato's dialogues is Socrates, a character who often speaks ironically. Other characters can be read as sometimes being ironical as well (such as The Athenian in the dialogue "Laws"). Any interpretation must determine when a character is speaking ironically and when seriously.
(3) Stories/Myths: Characters in Plato's dialogues often tell stories whose subject matter is mythological - they concern Gods and Goddesses, the afterlife, and other subject matter beyond ordinary human experience. Any interpretation that deals with them must determine how they are to be read.
(4) The Platonic Lie: In "The Republic", Plato endorses (or seems to endorse) lying as a means of instilling beneficial beliefs in audiences that are unable to acquire philosophical knowledge. A beneficial belief is one that is not true in its substance, but which, if believed, will tend to the same end as would the corresponding knowledge. If we accept that this is Plato's view, then interpretations must consider whether views expressed in the dialogues are themselves Platonic Lies, and not real representations of Plato's thought.
(5) Historical Background: Plato lived in a time and place different from our own, whose language, customs, intellectual background, and attitudes are not ours. This is a much bigger problem than just unfamiliar names - it is the unconscious attitudes we absorb from our culture (and he from his) of which we are not necessarily even consciously aware. Different interpreters do not read these influences the same way (there is no book we can all go to called "How We Thought About Things", authored by "The Ancient Greeks").
With regard to these issues, the dominant view in "A Cambridge Companion to Plato" is something I would call Platonic Fundamentalism: "Socrates says what Plato means, and he means what he says" (this is after the Christian Fundamentalist credo: "The Bible says what it means and it means what it says").
A difficulty with this view is that it leaves Plato contradicting himself an awful lot. The general solution presented here is the evolving-Plato theory - that the dialogues were written over a long period of time and that the contradictions represent real changes in Plato's views. The collection thus abounds in references to Plato's "early dialogues" or "middle dialogues" or "late dialogues".
Now, there are certainly Plato scholars, past and present, who do not accept this particular interpretive framework, but their views, if raised at all, are raised only so that they may be dismissed (sometimes in the same sentence). Those looking for substantial engagement on the problems of Platonic interpretation must look elsewhere.
So, given that the book does not aim to present the scholarly debate on interpreting Plato, it is fair to ask: what does it aim to do? This is an excellent question, but I could not find the answer to it in the book itself.
If it were for the beginning reader, I would think it would focus on the order of reading, and on prepping the reader with background info for each dialogue so as to make reading it more rewarding. But it doesn't do anything like that.
If it were for the intermediate reader, I would think it would focus on illuminating doubtful passages or drawing connecting webs across disparate ones. But it doesn't do that either.
If it were for the advanced reader, I would think it would focus on the debates in the secondary literature, and that it would be used by peers to address peers on controversies. But it doesn't do that either.
So, when it comes to the ultimate question of whether I should recommend the book or not, I just can't think of anyone to whom I would recommend it.
Finally, to take another tack at how worthwhile a book is: the basic challenge any work of secondary literature must face is whether it is more profitable to read it, or to give the primary literature another reading instead.
The only work in the collection that I would say clearly passes that test is Constance Meinwald's essay on "Parmenides" (for those who don't know, "Parmenides" is by far the most formidable work in the Platonic corpus - the first half works to demolish the theory of Forms that we might otherwise hold to be Plato's view, and the second half defies the ability of most readers to make any sense of at all). Even here, however, if you want to read Meinwald's theories on "Parmenides" (and they are worth reading), you would do better to get her book "Plato's Parmenides" than to read the essay excerpted from that book included here.