Until now we have spoken of the passive understanding, which is the nature of the soul, but also in equal degree its faculty of sense-perception and imagination. Aristotle now proceeds to distinguish active understanding from this, as follows (De Anima, III. 5): “In nature as a whole there is present in every species of things, on the one hand, matter, which in potentiality is the whole of this species, and, on the other hand, cause and energy, operative in all things, in the same way that art is related to matter. It therefore necessarily follows that in the soul also these different elements should be present. The faculty of understanding is thus, in one view of it, the capacity of becoming all things; but in another view it is the capacity of creating all things, as is done by an efficient power (ἕξις), light, for instance, which first causes the colours which exist in potentiality to exist in reality. This understanding is absolute (χωριστός), uncompounded, and not influenced from without, as it is essentially activity. For the active is always more in honour than the passive, and the principle more in honour than the matter that it forms. Knowledge, when in active exercise, is identical with the thing (πρᾶγμα) known; but what is in potentiality” (that is, external reason, imagination, sense-perception) “is certainly prior in respect of time in one and the same individual, but in the universal (ὅλως) it is not even so in respect of time. Active understanding is not such that it sometimes thinks and sometimes does not. When it is absolute, it is the one and only existence; and this alone is eternal and immortal. We, however, do not remember this process, because this understanding is unaffected from without; but the passive understanding is transitory, and without the former it is incapable of thought.”