Tantalising hints of a dialogue on the nature of the philosopher may be found at the beginning of Plato's Sophist (217a–b) and Statesman (257a–258b). Philosophos would have been the final instalment of a trilogy. There is prima facie evidence that such a dialogue was conceived, and the joint investigation of the Eleatic Stranger, Theaetetus and Young Socrates is advertised as culminating in an account of the philosopher as a discernible type, both distinct from and more important than the sophist and the statesman. Were a manuscript of Philosophos to be discovered in a forgotten desert cave it would ascend immediately to a prominent position among Plato's dialogues.

According to G. we should not hold out hope for such a discovery, for she thinks Plato never intended to write a dialogue named Philosophos. Rather, he left to readers the task of constructing an account of the philosopher's nature using evidence strategically disseminated in the series comprised of Parmenides, Theaetetus, Sophist and Statesman. Dramatic links between the Theaetetus, Sophist and Statesman, along with cross-referencing between all four of these dialogues, point to a portrait of the philosopher as an expert in dialectic who seeks to understand the nature of things and helps others do the same. G. also specifies the subject of a philosopher's investigations (the form of being), and the properly philosophical perspective on that subject (the structural core inside the nature every being, ‘enabling it to fit together with other beings outside its specific nature’, p. 240).

G. begins with the Parmenides, in the first part of which Socrates is forced to confront formidable theoretical difficulties that ensue when forms are conceived of as separate from the particulars that participate in them. Much of the rest of the dialogue consists of Parmenides going through a series of deductions about oneness; these are undergone in order to illustrate the sort of exercises that constitute philosophical training. After completing exercises on oneness, a philosophical novice is encouraged to repeat the procedure for being, sameness, difference and other general kinds. In the first four (of eight) deductions undertaken to investigate oneness, G. detects a general pattern: (1) deduce the implications of a positive hypothesis, that oneness is; (2) then deduce the implications of its negative hypothesis, that oneness is not; (3) then deduce the implications of a middle way, that oneness both is and is not; (4) then develop a critical assessment of step three. This pattern, G. maintains, is the template for an extended investigation of the form of being in the Theaetetus, Sophist and Statesman.

It is necessary to read the three dialogues as a trilogy contributing to a single project in order to follow the pattern G. outlines. (1) As part of its guiding investigation into knowledge, the Theaetetus explores the hypothesis about the nature of being; problems arise because Heraclitean instability is presupposed in its conception, but indications of Parmenidean stability suggest alternative routes of inquiry. (2) Because the sophist inhabits the realm of non-being, the Sophist investigates the hypothesis that non-being has a kind of being, explained there in terms of difference. (3) Still in the Sophist, the Battle of the Gods (who champion being in terms of stability and rest) and Giants (who champion it in terms of instability and change) is momentarily resolved by the children's plea: accept both together (249c–d; pp. 98–100, 205, 229–30). The resolution lasts only until the Eleatic Stranger convinces Theaetetus that change and rest are jointly exhaustive opposites that exclude one another; and while each of these opposites relates to being, the being of change and being of rest must lie outside the specific natures of both. On G.'s reading, the fourth step here is aporetic, and diagnosing the flaw in its formulation is crucial for understanding the philosopher's nature and subject. While non-being can be accounted for in terms of difference, being itself is more elusive. Finally, the Statesman points the way to differentiate the philosopher from the statesman and from other rivals.

Why did Plato not write a Philosopher? In G.'s opinion, the answer is primarily pedagogical. Withholding an explicit account of the philosopher encourages readers to refine their own dialectical capacities as they assemble the evidence. Furthermore, an account of the philosopher is not the only thing that needs to be discovered by carefully reading and rereading the Parmenides–Theaetetus–Sophist–Statesman series. A deep problem with the form of being runs throughout these dialogues. Readers must first uncover its presence, then work their way towards its resolution. The crux of this problem comes in the Sophist, where the Eleatic Stranger and Theaetetus posit an apparent but unreliable parity between being and non-being. They locate the sophist in the dark region of non-being and happen upon the philosopher in the light of being; when they solve the problem of non-being technically by analysing it in terms of difference, it appears at first that the problem of being and the nature of the philosopher can be elucidated in a comparably straightforward manner. Not so, argues G., for the spatial metaphors by which being is related to change and rest suggest misleadingly that change and rest are categorial opposites and that being is outside both of them (i.e. being is a separate third thing, 250a–d); the Aporia about Being results from treating being as an external attribute to both change and rest, a result that echoes the fifth deduction of oneness in Parmenides (p. 211). G. argues that, despite the initial treatment of change and rest as categorial forms, ultimately readers are supposed to regard them as structural forms (p. 228). It is essential for Plato's project that his audience comes to see for itself both the Aporia about Being and the evolving analysis that is indispensable for resolving it. All this camouflaging and signposting serves a pedagogical purpose that, according to G., would have been lost if Plato had simply written the Philosopher.

G.'s account of the form of being is patiently developed, and its wider implications are nicely explained in the final pages. The form of being provides each being with its internal stability, the enduring nature of which remains the same as it changes accidentally in its traffic with other things outside its nature. (No summary can do justice to G.'s detailed account, which takes up large parts of two chapters and an appendix.) Because being is structurally implicit in all beings, the philosopher, whose focus is on being as such, can be distinguished from specialists who study only a part of being (demarcated, e.g., as geometry, astronomy, medicine, etc., p. 241). And because the philosopher's work is guided by dialectic, this study is sharply differentiated from the sophist's unregulated indulgence in phantastikē (p. 243). Moreover, G. thinks that the models used in the Sophist and Statesman are more apt for the philosopher than for the sophist and statesman: angling has more in common with the pursuit of wisdom than the hunting of young men; and the weaver expresses more precisely the activity of weaving words into sentences, arguments and accounts than that of intermingling moderate and courageous elements of the polis (p. 243).

G.'s exposition and analysis meet a very high standard. Anyone with an interest in Plato will study Philosophos profitably. I am a little disappointed that she does not explain how her portrait of the philosopher compares with the philosopher in the Republic, and that she does not explore how her analysis of the form of being relates to Socrates' remark in Republic 6 that the good is prior to the forms and to being (509b). Perhaps these are matters for another occasion. But anyone who tackles these issues will be well served by G. and her very fine book.