As they recall the prophecy of Calchas and the sacrifice of Iphigeneia, the chorus of Agamemnon call upon Zeus, “whoever he is,” and address to him this puzzling expression: οὐκ ἔχω προσεικάσαι (Ag. 163). Before Peter Smith’s study of this passage, the object of wonder, for which the elders “cannot find the like,” was understood to be the god himself, despite the odd tautology that results.1 Fraenkel’s rendering is representative: “I have nothing whereto to liken him . . . nothing save Zeus” himself. But, as Smith showed, the referent is really the sequence of disastrous events, the sacrificial murder of the child and its aftermath: the meaning of προσεικάσαι is more like “find an identity (for it).”2 The Argive elders are wrestling with a problem in categorical thinking, and for them the only plausible character to put upon this chain of causation is Zeus. To find parallels for this expression, Smith’s study did not venture much beyond the proper bounds of archaic poetry, but in this essay I argue that it fits a persistent pattern in reasoning about agents and their actions, one more familiar from the later fifth century. The elders are trying to define the atrocity that looms over this story, a crime within the family for which justice seems elusive but retribution inevitable, and the only way they can make sense of it is by reasoning from “likeness” or “probability,” eikos.