(no subject)

A thing is called necessary either in respect to its essence or in respect to its cause; for the existence of a thing necessarily follows, either from its essence and definition, or from a given efficient cause. For similar reasons a thing is said to be impossible; namely, inasmuch as its essence or definition involves a contradiction, or because no external cause is granted, which is conditioned to produce such an effect; but a thing can in no respect be called contingent, save in relation to the imperfection of our knowledge.

A thing of which we do not know whether the essence does or does not involve a contradiction, or of which, knowing that it does not involve a contradiction, we are still in doubt concerning the existence, because the order of causes escapes us,—such a thing, I say, cannot appear to us either necessary or impossible. Wherefore we call it contingent or possible.

-- Benedict de Spinoza. Ethics, Part I, PROP. XXXIII.

http://www.gutenberg.org/files/3800/3800-h/3800-h.htm#chap01

A x B -> ∅