We take issue with Shorey's thesis that the closest analogy to the Aristotelian syllogistic can be found in the causal reasoning of the Phaedo 100ff and that his "first idea" comes from this doctrine of Idea's causality. In our assesment, Shorey ignores the basic difference in the problem situation which lies between the causality discussed in AP_0 and the logical necessity in APr. We argue that the origin of the "first idea" should be traced back to Aristotle's practical interest in winning the argument, which in turn has led Aristotle to a theory of valid inference. First, he develops it as the Topos theory in Top II-VII 2. The aim of this theory is to offer the lists of argument-forms or rules concerning the validity of propositions. This prefigures some moments of his syllogistic, such as rules of logical necessity or propositional implications, the theory of the square of oppositions. Based on this Topos theory, the dialectical syllogism is theorized to a certain extent in Top I, VII 3-VIII, SE. Finally, Aristotle moves from the dialectical syllogism to the formal system of the syllogistic theory in APr by utilizing some aspects of the proportional theory of Pythagorean mathematics especially Book V of Euclid's Elementa by Eudoxos. Eudoxos supplies Aristotle with most of terminologies and technical procedures. Thus, Logic told the dawn.