BUDDHĪ or buddhī (from Sanskrit budh--to wake up, be awake, to perceive, learn) is the intellectual aspect of mind (antahkaraṇa) whose other aspects man and haumai are intertwined with it in close interrelationship. Its nearest English equivalent may be intellect.
Man (Sanskrit manas) as the receptacle of sense-impressions from sense-organs, organizes them into precepts, yet it has doubt or indetermination about them. Buddhī defines and ascertains them and brings about definite and determinate cognition. Man simply assimilates sense-impressions; haumai (or ahahṅkāra) self-appropriates the apperceived impressions, while buddhī determines their nature, categorizes them and welds them into concepts. Its function, then, is to bring about certainty and definitiveness in knowledge. Definitive apprehension might spur action. Thus it is buddhī which resolves to act and then guides the ensuing action.
A fundamental categorization of percepts as also of ensuing actions concerns their moral import. The deftness with which buddhī does that is variable. If it can exercise acute ethical discrimination, it is known as bibek buddhī (discriminative intellect). That can happen only if it has become God centred. On the contrary, if it remains self-centred (aham buddhī), then it remains morally confounded and unable to discriminate.
Bibek buddhī in gurbāṇī, Gurū's utterance, has also been called sār-buddhī (the essential intellect), tat buddhī (the real intellect), bimal or nirmal buddhī (unclouded, clear intellect), bal buddhī (powerful intellect), mati buddhī (the counselling intellect) and sudh buddhī (pure intellect).
Aham buddhī has also been called chapal buddhī (the unstable intellect), buddhī bikār (foul intellect), malīn buddhī (turbid intellect), nibal buddhī (weak intellect), durmat buddhī (perverse intellect), and phaṇin buddhī (the deluding intellect).
This moral bipolarity of the functioning of intellect stands out in relief in gurbāṇī. In its decadent form, buddhī wastes itself in vain, egoistic pursuits : kaunu karam merā kari kari marai--for what reason does it die proclaiming mine ! mine ! ? (GG, 1159). However, when through evolution it ascends up the ethical scale (buddhī-pragās), it flowers into bibek buddhi which is a divine attribute : tū samrathu tū sarab mai tū hai buddhi bibek jīu - You are omnipotent, you are all pervasive, you are the discriminating intellect (GG, 761). However, if it begins to undergo the process of devolution (visarjan) down the moral scale, buddhī becomes delusional intellect (phaṇin buddhī).
Buddhī, also called akal (Arabic 'aql) in gurbāṇī, is considered to be an instrument for serving the Divine purpose and acquiring merit : akalī sāhibu sevīai akalī pāīai mānu - by wisdom is the Lord served; by intellect is honour attained (GG, 1245). By contrast, buddhī in its decadent form is not only infirm but also arrogant, which makes it despicable :
Some are devoid of intellect, or sense, or comprehension
And understand not a syllable.
Such folk, saith Nānak, as fill themselves with pride,
Without merit are asses pedigreed.
Jaswant Siṅgh Nekī