Some of them maintain, rather, that the demonstrative syllogism is constructed from necessary propositions, the dialectical from those that are true for the most part, the rhetorical from equipollent possibilities, and the poetical from impossibilities. Yet this reckoning is worthless, for the poetic syllogism is whatever the soul can sense; its purpose is to repel or attract, to inspire hope or fear. It may be true and it may be false, but what is sought in poetry is the stirring of the soul, not the imparting of knowledge.
Ibn Sīnā rejected this view, saying:
“Do not be distracted by the claim that demonstrative propositions are necessary, dialectical propositions merely possible in most cases, rhetorical propositions equally possible, lacking example or force, and poetic propositions false and impossible. That is not the criterion, and the Master of Logic never hinted at it. Nonetheless, while refuting that view, he reported a second opinion: demonstrative syllogisms, he said, are composed of premises whose acceptance is obligatory; dialectical syllogisms of widely known propositions, whether those propositions be necessary, possible, or impossible; rhetorical syllogisms of conjectural and accepted notions that are not famous, and the like, even if impossible; poetic syllogisms of imaginatively suggestive premises inasmuch as they affect the one upon whom the image is cast, whether those premises be true or false. Sophistical syllogisms, by contrast, employ specious arguments, and those who test or probe in order to deceive share in that practice. If the resemblance lies in the use of necessary premises, the practitioner is called a sophist; if in the use of well-known premises, he is labelled a quibbler and wrangler. The quibbler corresponds to the dialectician, and the sophist to the sage.”
What he set down here concerning the material of the dialectical and rhetorical syllogisms is likewise weak. The true material of the dialectical syllogism is whatever the disputant concedes, whether famous or obscure, true or false. The material of the rhetorical syllogism is the widely accepted and the like with which the common folk are addressed. Each of these may be demonstrative or non-demonstrative, true or false. This was the intention of the early masters, and it accords more closely with their terminology and division.
As for the view that rhetorical premises are, without exception, conjectural, that is a mistake according to them; for if dialectical premises may sometimes yield certain knowledge, then rhetorical premises—which in their judgment hold a higher rank—are even more entitled to yield certain knowledge, for