People may prefer what merely appears just or honourable, but no one is content with what merely appears good. Rather, Socrates claims at Republic 505d11ff., the truly good is:

Socrates declines, at Rep. 506d6–507a5, to give a straightforward account of the Good. Moreover, we learn that only a select few individuals will ever make the full intellectual ascent to apprehend the Good. We are left, then, with some pressing questions: how can the Good be the end of all human action when most of us do not adequately grasp its nature? How does one aim at the Good when one lacks explicit and accurate beliefs and desires concerning it? How can the Good order one's life or order a political community while we remain in the dark about it?

In response to such questions, P.’s book knits together parts of the Republic that are often treated separately by commentators. The unifying thread in P.’s interpretation is functional teleology. For P., functional teleology concerns actions directed at an end not explicitly intended by the agent. In intentional teleology, by contrast, the end is explicitly represented in the agent's beliefs and desires. So, a doctor acts towards the end of improving health, and this end is captured in the content of her beliefs and desires. She desires to improve the health of the patient and believes that this treatment regimen will have the desired effect.

Functional teleology is also distinct from natural teleology. In natural teleology certain phenomena or behaviours are explained as purposive, even in absence of any relevant beliefs or desires. The spider constructs a web for the sake of catching food, though it has no explicit intentions towards this end. In contrast, actions on a functional teleological model go through, as it were, beliefs and desires, but the beliefs and desires do not explicitly reference the end in question.

P. derives the core concept of functional teleology from the ascent passage in Plato's Symposium 210a–212a. Here the lover ascends through a series of activity stages culminating in the vision of Form of the Beautiful. For P., the lover cannot have the Beautiful as an explicit end prior to achieving the final stage of the ascent. Rather, the lover's progression is best explained as aimed at the unintended end of eternally possessing good and beautiful things.

Function enters the picture as the crucial link between the lover's actions and the unintended end. It is the function of eros, as Diotima proclaims, to ‘give birth in the beautiful’ (206e). P. understands this mysterious claim to indicate that the lover, at every stage in the ascent, engages in activities that express eros’ function. Eros’ function has as its ultimate end eternally possessing good and beautiful things. Thus, on P.’s account, eternally possessing good and beautiful things guides the lover's actions as an unintended end, even if the lover's actions are not directly motivated by beliefs and desires that explicitly reference the Good or the Beautiful.

Following Chapter 2, P. extends the framework of functional teleology to the Republic. As P. has it, functional teleology undergirds certain of the Republic’s key arguments and theoretical commitments. Many commentators have regarded Rep. 1 as little more than an aporetic ground-clearing exercise. P. finds more interest in the opening book of the Republic. For one, P. holds that Rep. 1 points the way towards a distinctive ergon for justice, that of forming and maintaining partnerships and associations (p. 37).

Furthermore, rather than understanding justice as a craft in Rep. 1, P. argues that justice is better understood as a ‘self-transmitting power’ (pp. 40–1), after the general definition of powers at 478c1–d5. Justice, that is, is similar to heat. The just person transmits justice to others through the function of justice by engaging in associations that cause others to become more just. Concord and friendship are unintended ends of these associations. So, as P. has it, the just person can both be guided by these unintended ends, and just actions can produce concord and friendship as results – but the just person need not have these ends clearly in view (p. 71).

In Chapters 5–7, P. employs functional teleology to address several daunting problems that threaten the Republic’s defence of justice, account of the soul and of moral psychology. Space does not permit a full discussion of the problems or P.’s proposed solutions. But I will briefly discuss one example – the much-discussed problem of the gap between psychic justice and just action, noted by D. Sachs, T. Irwin and S. Waterlow Broadie. The crux of the problem is this: Socrates is challenged to defend justice in the sense of conventionally just actions, including other-regarding actions; what Socrates apparently defends instead is having a just soul; arguably, Socrates provides no convincing link between psychic justice and conventional just actions. Hence, the gap.

P. acknowledges that, on the Republic’s account, the just person will have an explicit concern to create and preserve harmony in her own soul. The just person may lack an explicit concern to promote others’ good in some particular action. However, P. allows that the just person has the good of others as an unintended end of her just actions (pp. 139–49). Justice's ergon ensures that acting to improve harmony in one's own soul will promote the good of others. On P.’s view, functional teleology successfully bridges the gap between psychic justice and just action.

A notable highlight of P.’s fine book is its detailed and careful treatment of Rep. 6–7. After higher studies in maths and astronomy, the philosopher-ruler's education culminates in dialectic. While advanced maths has the effect of turning the soul away from the transitory world of becoming, only dialectic ‘does away with hypotheses’ and advances to ‘a first principle’ (533c–d). How dialectic accomplishes the move to unhypothetical first principles, however, has provided ample matter for scholarly debate.

P. argues that the Good guides the higher studies as an unintended end. Practicing mathematicians have an explicit concern with properly mathematical entities, problems and methods. They do not attempt to account for the hypotheses and definitions that provide their starting points. It falls to the dialectician to investigate mathematical hypotheses and definitions, and, in doing so, to attain a comprehensive understanding of commensurability, unity and harmonious order. In his interpretation, P. marks a sharp distinction between the Republic’s account of dianoia and its account of dialectic. P.’s interpretation thus provides an important counterweight to interpretations, such as H. Benson's, which argue for strong continuity between dianoia and dialectic.

I close with two reservations, which are more on the order of requests for further development. As may be already evident, functional teleology is an extremely flexible concept with broad applicability. But teleological accounts always run the risk of defaulting to ‘just so’ stories with little explanatory worth. So, I worry that functional teleology's power may also be its weakness. In future work, I hope that P. provides a fuller defence of functional teleology on this score. Second, I have some qualms about P.’s reading of the Republic’s psychology. In Chapter 6, P. characterises the soul's parts as agent-like. He further posits that the soul's unity is an unintended end for the psychic parts. However, there are well-known difficulties with attributing agent-like status and independent beliefs, desires and actions to the soul's parts. I invite P. to more fully address these worries.

There is much of interest and value in P.’s book for specialists. I believe that other scholars, graduate students and advanced undergraduates with a strong interest in Plato's work will also find P.’s book a valuable resource. P.’s book is wide-ranging, treats textual issues with care and engages productively with the scholarly literature on important questions. P.’s book also achieves that rare feat – a novel perspective on some of the Republic’s most perennial problems.