Page:O. F. Owen's Organon of Aristotle Vol. 1 (1853).djvu/143

 but B is neither contingently present, nor yet contingently not present, with C. It is evident that it is not contingently present, for no "horse" is "a man," but neither does it happen not to be present, for it is necessary that no "horse" should be "a man," and the necessary is not the contingent, wherefore there is no syllogism. This may be also similarly shown, if the negative be transposed, and if both propositions be assumed affirmative, or negative, for the demonstration will be by the same terms. When one proposition also is universal, but the other particular, or both particular or indefinite, or in whatever other way it is possible to change the propositions, for the demonstration will always be through the same terms. Hence it is clear that if both propositions are assumed contingent there is no syllogism.

Chapter 18
one proposition signifies inesse, but the other the contingent, the affirmative proposition being simple, but the negative contingent, there will never be a syllogism, neither if the terms be