"Eidos and Diaeresis in Plato's Statesman," Philologus 107, nos. 3-4 (1963): 193-226. Reprinted in The Archaeology of the Soul, 2012.
When the Stranger summarizes the set of divisions which presents the statesmen as a ruler of two-footed swine, he omits the differentia, even though he had gone to some trouble to introduce it. According to Campbell, this imprecision and another related one at 266e4-11 might not arise from neglect: “they are caused by Plato’s instinctive avoidance of an over-exactness, which would be unnatural in conversation.” Such an avoidance might be thought suitable for the Republic, but not one, not even Campbell elsewhere, has ever seen the Sophist and Statesman as anything but highly artificial dialogues, where they style and subject-matter hardly look like “natural” conversation. That Plato’s instinct should overtake him on so small a point but desert him almost everywhere else, seems to make his artistry too fitful to be considered artistry at all. It would be far better to mark it as a mistake than to assume that Plato now and again reverted to the apparent laxity of his other dialogues, so that his readers would know he still had the skill to imitate natural speech. But we are not forced to choose between error and instinct: the discussion itself explains its omission.