The doctrine of space and time as forms of sense-perception, the reference of both space and time and the pure intellectual notions to the laws of the activity of mind itself, the distinction between sense and understanding as one of kind, not of degree, with the correlative distinction between phenomena and noumena, - all of these reappear, though changed and modified, in the Kritik.
Accordingly, when he denies the reality of Being, and declares Becoming, or eternal flux and change, to be the sole actuality, Heraclitus must be understood to enunciate not only the unreality of the abstract notion of being, except as the correlative of that of not-being, but also the physical doctrine that all phenomena are in a state of continuous transition from non-existence to existence, and vice versa, without either distinguishing these propositions or qualifying them by any reference to the relation of thought to experience.
But if these three correlative facts are immediately given, it seems to be thought possible by Cousin to vindicate them in reflective consciousness.
Under the title of essence are discussed those pairs of correlative terms which are habitually employed in the explanation of the world - such as law and phenomenon, cause and effect, reason and consequence, substance and attribute.
The idea of necessity is not intuitively obvious; the ideas of cause and effect are correlative in our minds, but only as a result of experience.
This test is roughly of two kinds, first by the ultimate principles or presuppositions on which a particular branch of knowledge rests, and second by the comparison of correlative facts.
On the other hand, the world can be known as the realm of antithesis, and it is the correlative of God.
Max Muller used it particularly in connexion with the Vedanta philosophy for the correlative of ignorance or nescience (Gifford lectures, 1892, c. ix.).
The idea of the world as the totality of being is, like the correlative idea of God, only of regulative value; it is transcendent, as we never do more than make approaches to a knowledge of the sum of being.
This is important as a preliminary stage, but philosophy properly begins when it attempts to coordinate or systematize those convictions in harmony, to conciliate apparent contradiction and opposition, as between the correlative notions of finite and infinite, the apparently conflicting notions of personality and infinitude, self and not-self; in a word, to reconcile the various sides of consciousness with each other.