Indriya (literally "belonging to or agreeable to Indra") is the Sanskrit and Pali term for physical strength or ability in general, and for the senses more specifically. The term literally means "belonging to Indra," chief deity in the Rig Veda and lord of the Trāyastriṃśa heaven (also known as Śakra or Sakka in Buddhism) hence connoting supremacy, dominance and control, attested in the general meaning of "power, strength" from the Rig Veda.
In Buddhism, the term refers to multiple intrapsychic processes and is generally translated as "faculty" or, in specific contexts, as "spiritual faculty" or "controlling principle." In Buddhism, depending on the context, indriya traditionally refers to one of the following groups of faculties: the 5 spiritual faculties the 5 or 6 sensory faculties the 22 phenomenological faculties
5 spiritual faculties
In the Pali Canon's Sutta Pitaka, indriya is frequently encountered in the context of the "five spiritual faculties" (Pali: ): faith or conviction or belief () energy or persistence or perseverance (viriya) mindfulness or memory (sati) stillness of the mind () wisdom or understanding or comprehension (). Together, this set of five faculties is one of the seven sets of qualities lauded by the Buddha as conducive to Enlightenment.
SN 48.10 is one of several discourses that characterizes these spiritual faculties in the following manner: Faith/conviction is faith in the Buddha's awakening. Energy/persistence refers to exertion towards the Four Right Efforts. Mindfulness refers to focusing on the four satipatthana. Stillness of the mind refers to achieving the four jhanas. Wisdom/understanding refers to discerning the Four Noble Truths.
In SN 48.51, the Buddha declares that, of these five faculties, wisdom is the "chief" (agga).
Balancing the spiritual faculties
In AN 6.55, the Buddha counsels a discouraged monk, Sona, to balance or "tune" his spiritual faculties as one would a musical instrument: "... what do you think: when the strings of your [lute] were neither too taut nor too loose, but tuned to be right on pitch, was your [lute] in tune & playable?"
"Yes, lord."
"In the same way, Sona, over-aroused persistence leads to restlessness, overly slack persistence leads to laziness. Thus you should determine the right pitch for your persistence, attune the pitch of the [five] faculties [to that], and there pick up your theme."
Relatedly, the Visuddhimagga and other post-canonical Pali commentaries caution against one spiritual faculty overpowering and inhibiting the other four faculties, and thus generally recommend modifying the overpowering faculty with the investigation of states (see dhamma vicaya) or the development of tranquillity (samatha). Moreover, these commentaries especially recommend that the five spiritual faculties be developed in counterbalancing dyads:
"For one strong in faith and weak in understanding has confidence uncritically and groundlessly. One strong in understanding and weak in faith errs on the side of cunning and is as hard to cure as one sick of a disease caused by medicine. With the balancing of the two a man has confidence only when there are grounds for it." (Vism. Ch. IV, §47, ¶1) "... [I]dleness overpowers one strong in concentration and weak in energy, since concentration favours idleness. Agitation overpowers one strong in energy and weak in concentration, since energy favours agitation. But concentration coupled with energy cannot lapse into idleness, and energy coupled with concentration cannot lapse into agitation. So these two should be balanced ; for absorption comes with the balancing of the two." (Vism. Ch. IV, §47, ¶2) "... One working on concentration needs strong faith, since it is with such faith and confidence that he reaches absorption." (Vism. Ch. IV, §48) "... Then there is [balancing of] concentration and understanding. One working on concentration needs strong unification, since that is how he reaches absorption; and one working on insight needs strong understanding, since that is how he reaches penetration of characteristics; but with the balancing of the two he reaches absorption as well." (Vism. Ch. IV, §48) The commentator Buddhaghosa adds: "Strong mindfulness, however, is needed in all instances; for mindfulness protects the mind lapsing into agitation through faith, energy and understanding, which favour agitation, and from lapsing into idleness through concentration, which favours idleness." (Vism. Ch. IV, §49).