As for satya a thing is said to be satya, true, when it does
not change the nature that is ascertained to be its own; and a thing is said to
be unreal when it changes the nature that is ascertained to be its own. Hence a
mutable thing is unreal … So the phrase satyam brahma (Brahman is truth)
distinguishes Brahman from unreal things.
From this it may follow that (the unchanging) Brahman is the
(material) cause (of all subsequent changes); and since a material cause is a substance,
it can be an accessory as well, thereby becoming insentient like earth. Hence
it is said that Brahman is jnānam. Jnāna means knowledge, consciousness. The
word jnāna conveys the abstract notion of the verb (jnā, to know); and being an
attribute of Brahman along with truth and infinitude, it does not indicate the agent
of knowing.
Source - Acharya Shankara’s commentary on Taittiriya Upanishad,
2.1.1. Translation from Eight Upaniṣads,
with the Commentary of Śankarācārya, trans. Swami Gambhirananda, 2 vols
(Kolkata: Advaita Ashrama, 2006), 1.308–9.
Other Quotes on the Subject
It (i.e. Brahman) is really known when It is known with
(i.e. as the Self of ) each state of consciousness, because thereby one gets
immortality. (Since) through one’s own Self is acquired strength, (therefore)
through knowledge is attained immortality. (Kena Upanishad)
It is this heart (intellect) and this mind ... It is sentience,
rulership, secular knowledge, presence of mind, retentiveness,
sense-perception, fortitude, thinking, genius, mental suffering, memory,
ascertainment, resolution, life-activities, hankering, passion, and such
others. All these verily are the names of Consciousness. …
This One is (the inferior) Brahman; this is Indra, this is
Prajāpati; this is all these gods; and this is these five elements … and this
is all these (big creatures), together with the tiny ones, that are the
procreators of others and referable in pairs … and all the creatures that there
are which move or fly and those which do not move. All these are impelled by
Consciousness; all these have Consciousness as the giver of their reality; all
these; the universe has Consciousness as its eye, and Consciousness is its end.
Consciousness is Brahman. (Aitareya Upanishad)