Samkhya Karika 35

सान्तःकरणा बुद्धिः सर्वं विषयमवगाहते यस्मात् ।
तस्मात् त्रिविधं करणं द्वारि द्वाराणि शेषाणि ॥ ३५ ॥

sāntaḥkaraṇā buddhiḥ sarvaṁ viṣayamavagāhate yasmāt ।
tasmāt trividhaṁ karaṇaṁ dvāri dvārāṇi śeṣāṇi ॥ 35 ॥

sāntaḥkaraṇāḥ=along with other internal organs; buddhiḥ=intellect; sarvaṁ=all; viṣayam=objects of perception; avagāhate=apprehends; yasmāt=because; tasmāt=therefor; trividhaṁ=three-fold; karaṇaṁ=instruments; dvāri=door-keepers; dvārāṇi=doors; śeṣāṇi=remaining

Since the intellect, along with the other internal organs (mind, ego), apprehends all objects, these three act like the gate-keepers whereas the rest of the organs (organs of action and perception) are the gates.

The commentators have almost nothing to say about this karika. The karika simply states that along with the other two internal organs – mind and the ego – the intellect (buddhi) apprehends information related to all the objects presented to it through the ten external organs. In that sense, the ten external organs are presented as “gates”. The three internal organs are then labeled as “gatekeepers”. Since the three internal organs perform the job of apprehension, some commentators have labeled them as “principal”.