Wareh, Tarik. 2013. The Theory and Practice of Life: Isocrates and the Philosophers. Hellenic Studies Series 54. Washington, DC: Center for Hellenic Studies. http://nrs.harvard.edu/urn-3:hul.ebook:CHS_WarehT.The_Theory_and_Practice_of_Life.2012.
Chapter 2. Plato’s Concession to the Practical Arts in the Phaedrus
The import of this analogy is not, we will see, that medicine had a unique and distinct kind of claim to scientific rationality (as the least “popular” of the arts, to use a term from the Phaedrus). Rather, for Plato’s contemporary audience, who would have been better informed than we are about the methodological debates of the practical arts, the medical analogy involved Socrates in a host of issues that applied in common to multiple practical arts, including rhetoric as well as medicine. It is largely “the result of chance” that the first general epistemological critique in our literary record (the Hippocratic Art) is medical and not rhetorical, and it can be difficult to determine which of the two disciplines pioneered a given line of attack on their shared problems. 
Knowledge and Performance in the Phaedrus and in Isocrates
While introducing uncertainty about rhetoric’s status as a tekhnē, Socrates nonetheless considers that rhetoric’s defense may be satisfactory and admits that the better sort of existing rhetoric could have pretensions quite near to those he will advance for true rhetoric: not that it is an epistēmē, but that it depends on sound knowledge as a preliminary.
In this emphatic assertion (βούλομαι … ἔτι σαφέστερον εἰπεῖν … φημὶ γὰρ ἐγὼ …), Isocrates specifies the object of rhetorical knowledge as the “forms” (ideai) used in composition. Moreover, he insists that to gain such knowledge we must put ourselves in the hands not of the charlatans with their boastful promises, but of those who already possess some knowledge in this area (tois eidosi ti peri autōn).
Isocrates’ expression describing the value of the knowable ideai (ἐξ ὧν τοὺς λόγους ἅπαντας καὶ λέγομεν και συντίθεμεν) could denote anything from a very mechanical to a very sophisticated process of applying knowledge in preparation and performance.  We should remember both the sweepingly powerful definition Isocrates gives to logoi, and also that the conclusion of Socrates and Phaedrus’ discussion of the artless features of Lysias’ speech (atekhna, 262c6) focuses on its lack of principles of rational and organic construction (264b3–c5).  Even if we apply a fairly technical and limited definition of Isocrates’ ideai, it seems plausible that in this passage and others Isocrates is laying claim to a more organized compositional process that can defend itself at least against the charge of this particular kind of atekhnia.  In short, we are not entitled to be as surprised and skeptical as many would be when faced with the statement of Sextus Empiricus (Adversus rhetoras 62), who in a survey of various definitions of rhetoric notes that, whereas one Athenaeus defines it more conventionally as logōn dunamis,  “Isocrates asserts that the practice and pursuit of orators is no other thing than the knowledge or science [epistēmē] of persuasion.” 
This half of the equation is quite reminiscent of Isocrates’ ideai, which in the same passage as above he too calls the eidē of logoi:
Socrates uses the kairos terms favored by Isocrates three times in close succession,  clearly talks about how the speaker will apply the kinds of effects any interpreter would include among the Isocratean ideai, and very significantly finds in this performative culmination of the student’s training the decisive proof that “the tekhnē is well and completely produced” in him. 
This last example pushes the pattern back into the fifth century, confirming that the Isocratean language that we can show was topical for Plato and Aristotle has its roots in the earlier rhetorical and tekhnē tradition.
Ober has pointed out the Platonic and Socratic overtones of this. 
Adoleskhia kai meteo̅rologia
The Art of Medicine
In fact, Schiefsky comes to the conclusion that “only in VM do we find an explicit recommendation of something like the method Socrates describes as a method of investigating human φύσις,” so that, “If Plato had in mind any of the texts that make up the present Hippocratic Corpus, surely it was VM,” notwithstanding the evident differences between the two theories. 
Indeed, I suggest that this methodological ambivalence is more generally appropriate to the practical arts (in particular, Isocratean rhetoric), and we have seen that we must take very seriously Plato’s engagement with this kind of methodology in the Phaedrus.
Not Every Practical Art: The Counterexample of Harmonics