The judgment, " not-being is thinkable," cited by Aristotle; the judgment, " A square circle is impossible," cited by Herbart; the judgment, " A centaur is a fiction of the poets," cited by Mill.
- As Aristotle remarked both in the De Interpretatione and in the Sophistici Elenchi, " not-being is thinkable " does not mean " not-being exists."
Of such primitive principles, the absolutely necessary conditions of possible cognition, only three are thinkable - one perfectly unconditioned both in form and matter; a second, unconditioned in form but not in matter; a third, unconditioned in matter but not in form.
But some conceptions are such that the more distinct they are made the more contradictory their elements become; so to change and supplement these as to make them at length thinkable is the problem of the second part of philosophy, or metaphysics.
The cardinal assumption of Plato's metaphysic is, that the real is definitely thinkable and knowable in proportion as it is real; so that the further the mind advances in abstraction from sensible particulars and apprehension of real being, the more definite and clear its thought becomes.