Buddhi (Sanskrit: बुद्धि) refers to the intellectual faculty and the power to "form and retain concepts, reason, discern, judge, comprehend, understand".
[1] Buddhi means, states Monier Williams, the power to "form, retain concepts; intelligence, reason, intellect, mind", the intellectual faculty and the ability to "discern, judge, comprehend, understand" something.
[2][3][4] In Sankhya and yogic philosophy both the mind and the ego are forms in the realm of nature (prakriti) that have emerged into materiality as a function of the three gunas (गुण) through a misapprehension of purusha (पुरूष) (the consciousness-essence of the jivatman).
Discriminative in nature (बुद्धि निश्चयात्मिका चित्त-वृत्ति), buddhi is that which is able to discern truth (satya) from falsehood and thereby to make wisdom possible.
[5] In the Yoga Sutra, it is explained that the buddhi cannot illuminate itself, since it itself is the object of sight, "na tat svhāsam draśhyatvāt".