Brahma Knowledge, by L. D. Barnett, [1911], at sacred-texts.com

§ 11. Brahma is Thought.—The Vedic brahma, "prayer" or "spell," is naturally a function

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of intellect; and when it had risen to the rank of a cosmic Force, it retained this character. It is the universal subject of thought; but as it is itself the universe, and there is nothing beside, it is also its own object, like the Aristotelian νόησις νοήσεως; and as it is above the conditions of space, time, and causality, we can say of it only that it exists, and is Thought.

Frequently, from the natural tendency to conceive a higher sphere of existence as a realm of light and thought as itself light, Brahma and the individual Self identified with it are described as supreme self-luminous light: B.A. IV. iv. 16, Ch. VIII. iii. 4, XII. 3, Kaṭh. v. 15, Śvet. VI. 14.