Buddhi (Sanskrit, “intellect”) in Sanskrit (Hindu) literature is the higher mental faculty, the instrument of knowledge, discerment, and decision. Buddhi is comprehended slightly different in different philosophical systems.
Buddhi, however, is a higher faculty that acts in sense percepts organized by manas and furnishes intellectual discrimination, determination, reason, and will. As such buddhi is at the very core of one’s being, as sentient creatures, and the closest mental faculty to the atman, real Self or spirit.
In Samkhya-yoga philosophy, buddhi (or mahat, “the great one”) plays a key role. Buddhi is the first principle derived from unmanifest, prakrti (and predominant in sattva guna, (“intellectual stuff”), virtually transparent reflector for pure consciousness (purusa), with which buddhi mistakenly identifies. With this mistaken identification with the conscious principle, a fall into ignorance, buddhi produces the next principle, ahamkara, which in turn produces manas.
The three together make up the “internal instrument,” or antahkarana. For salvation, buddhi must attain the discriminative discernment between itself as unconscious matter, prahrti, and the independent and transcendent principle of pure consciousness, purusa. A.G.H.
Bowker, John, The Oxford Dictionary of World Religions, New York, Oxford University Press, 1997, p. 171