Indriya, literally "belonging to or agreeable to Indra" is the Sanskrit and Pali term for physical strength or ability in general, and for the five senses more specifically. 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." The term literally means "belonging to Indra," chief deity in the Rig Veda and lord of Tāvatiṃsa heaven, hence connoting supremacy, dominance and control, attested in the general meaning of "power, strength" from the Rigveda.
In Buddhism, depending on the context, indriya traditionally refers to one of the following groups of faculties:
- the "Five Spiritual Faculties"
- five or six sensory faculties
- 22 phenomenological faculties.
Five Spiritual Faculties 
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.
- Concentration 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).
Relation to the Five Powers 
In SN 48.43, the Buddha declares that the Five Spriritual Faculties are the Five Powers and vice-versa. He uses the metaphor of a stream passing by a mid-stream island; the island creates two streams, but the streams can also be seen as one and the same. The Pali commentaries remark that these five qualities are "faculties" when used to control their spheres of influence, and are "powers" when unshakeable by opposing forces.
5 Material or 6 Sensory Faculties 
- vision (cakkh-indriya)
- hearing (sot-indriya)
- smell (ghān-indriya)
- taste (jivh-indriya)
- touch (kāy-indriya)
- thought (man-indriya)
The first five of these faculties are sometimes referenced as the five material faculties (e.g., pañcannaṃ indriyānaṃ avakanti).
22 Phenomenological Faculties 
- six sensory faculties
- eye/vision faculty (cakkh-undriya)
- ear/hearing faculty (sot-indriya)
- nose/smell faculty (ghān-indriya)
- tongue/taste faculty (jivh-indriya)
- body/sensibility faculty (kāy-indriya)
- mind faculty (man-indriya)
- three physical faculties
- femininity (itth-indriya)
- masculinity (puris-indriya)
- life or vitality (jīvit-indriya)
- five feeling faculties
- physical pleasure (sukh-indriya)
- physical pain (dukkh-indriya)
- mental joy (somanasa-indriya)
- mental grief (domanass-indriya)
- equanimity (upekhha-indriya)
- five spiritual faculties
- three final-knowledge faculties
- thinking "I shall know the unknown" (anaññāta-ñassāmīt-indriya)
- gnosis (aññ-indriya)
- one who knows (aññātā-vindriya)
According to the post-canonical Visuddhimagga, the 22 faculties along with such constructs as the aggregates, sense bases, Four Noble Truths and Dependent Origination are the "soil" of wisdom (paññā).
Other faculty groupings 
At times in the Pali Canon, different discourses or Abhidhammic passages will refer to different subsets of the 22 phenomenological faculties. Thus, for instance, in the Abhidhamma there are references to the "eightfold form-faculty" (aṭṭhavidhaṃ indriya-rūpaṃ) which includes the first five sensory faculties (eye, ear, nose, tongue and body faculties) plus the three physical faculties (femininity, masculinity and vitality).
See also 
- Ayatana (sense base)
- Bodhi (awakening, enlightenment)
- Bodhipakkhiyadhamma (37 enlightenment qualities)
- Five Powers
- Four Right Efforts
- Prajna (wisdom)
- Salayatana (six sense bases)
- Bodhi (2000) translates indriya as "spiritual faculty" and, at times (particularly when referring to Abhidhammic sources), "faculty." Buddhaghosa & Ñāṇamoli (1999) consistently translate indriya simply as "faculty" both in the context of the five spiritual faculties (e.g., pp. 128-9) and the 22 phenomenological faculties (Ch. XVI). Conze (1993) mentions and uses translations of "faculty," "controlling faculty" and "spiritual faculty," and refers to the five indriya as "cardinal virtues." Thanissaro (1998) uses "faculty." Rhys Davids & Stede (1921-25), p. 122-123, entry for "Indriya," (retrieved 2007-05-27) defines it as: "Indriya is one of the most comprehensive & important categories of Buddhist psychological philosophy & ethics, meaning 'controlling principle, directive force, élan, dynamis'...: (a) with reference to sense-perceptibility 'faculty, function'...."
- Indra is known as Sakka in the Pali Canon.
- Bodhi (2000), p. 1509; Conze (1993), n. 1; Rhys Davids & Stede (1921-25), p. 122, entry "indriya"; and, Thanissaro (1998), Part II, sec. E, "The Five Faculties."
- While the Pali commentaries consistently use the term bodhipakkhiyā dhammā ("states conducive to enlightenment") to refer to seven sets of enlightement qualities (i.e., the four frames of reference, four right exertions, four bases of power, five faculties, five powers, seven bojjhanga, and Noble Eightfold Path) (see, e.g., Bodhi, 2000, p. 1937, n. 235), a search of the Sinhala SLTP tipitaka (using La Trobe University's search engine at http://www.chaf.lib.latrobe.edu.au/dcd/pali.htm) finds the Pali phrase bodhipakkhiyā dhammā occurring only once in the early suttas: in the Sālā Sutta (SN 48.51) where the term references solely these five spiritual faculties of faith, energy, mindfulness, concentration and wisdom (Bodhi, 2000, p. 1695).
- Alternatively, SN 48.8 and AN V.15 identify "faith" as referring to the four-fold faith of the stream-enterer which Conze (1993), n. 28, and Nyanaponika & Bodhi (1999), p. 297, n. 9, identify as faith in the Triple Gem and "perfect morality."
- Bodhi (2000), pp. 1671-73; and, Thanissaro (1997a).
- Bodhi (2000), p. 1695.
- Thanissaro (1997b). See also Nyanaponika & Bodhi (1999), pp. 168-70. Following Nyanaponika & Bodhi, the Pali word vīṇā (which Thanissaro leaves untranslated) is translated here as "lute"; other square-bracketed phrases are from Thanissaro (1997b). In Nyanaponika & Bodhi (1999), they translate this excerpt's last line as: "Therefore, Soṇa, keep your energy in balance, penetrate to a balance of the spiritual faculties, and there seize your object." In the associated end note (pp. 301-2, n. 31), they provide the commentary's interpretation of "object" (nimitta) as: "When such balance exists, the object can arise clearly, just like the reflection of the face in a mirror; and you should seize this object, be it of tranquillity, insight, path or fruition."
- See also the Aggi Sutta ("Fire Discourse," SN 46.53) in which, within the context of the seven enlightenment factors, the Buddha counsels that one should develop energy (and other factors) when experiencing a sluggish mind and develop concentration (and other factors) when experiencing an excited mind (Bodhi, 2000, pp. 1605-7).
- For instance, in an end note associated with AN 6.55, Nyanaponika & Bodhi (1999, pp. 301-2, n. 31) reference the Aṅguttara Aṭṭhakathā (AN commentary).
- Direct quotes from the Visuddhimagga are from Buddhaghosa & Ñāṇamoli (1999), pp. 128-9. Also mentioned in Bodhi (2000), p. 1511; and, Conze (1993), Part II, sec. 5, "The Balance of the Faculties."
- Bodhi (2000), pp. 1688-89.
- Bodhi (2000), p. 1511.
- Rhys Davids & Stede (1921-25), pp. 122-23.
- Bodhi (2000), pp. 1508-1509, refers to these 22 faculties as "phenomenological faculties"; while Rhys Davids & Stede (1921-25), p. 122-3, entry on "indriya" refers to these 22 faculties as "controlling powers."
- The five feeling faculties are essentially an expanded scale of the three vedana, where pleasant and unpleasant feelings/sensations are divided between physical and mental experiences (see, e.g., Bodhi, 2000, p. 1510).
- Buddhaghosa & Ñāṇamoli (1999), pp. 442-443.
- See, for instance, Dhs. 709-717, 971-973 (Rhys Davids, 2003, pp. 215-217, 247); and, Rhys Davids & Stede (1921-25), pp. 122-123.
- Bodhi, Bhikkhu (trans.) (2000). The Connected Discourses of the Buddha: A Translation of the Samyutta Nikaya. Boston: Wisdom Publications. ISBN 0-86171-331-1.
- Buddhaghosa, Bhadantacariya & Bhikkhu Ñāṇamoli (trans.) (1999). The Path of Purification: Visuddhimagga. Seattle, WA: BPS Pariyatti Editions. ISBN 1-928706-00-2.
- Conze, Edward (1980, 1993). The Way of Wisdom: The Five Spiritual Faculties (The Wheel Publication No. 65/66). Kandy: Buddhist Publication Society. Retrieved on 2007-05-27 from "Access to Insight" at: http://www.accesstoinsight.org/lib/authors/conze/wheel065.html.
- Nyanaponika Thera & Bhikkhu Bodhi (trans.) (1999). Numerical Discourses of the Buddha: An Anthology of Suttas from the Anguttara Nikaya. Kandy, Sri Lanka: Buddhist Publication Society. ISBN 0-7425-0405-0.
- Rhys Davids, Caroline A. F. (, 2003). Buddhist Manual of Psychological Ethics, of the Fourth Century B.C., Being a Translation, now made for the First Time, from the Original Pāli, of the First Book of the Abhidhamma-Piṭaka, entitled Dhamma-Sangaṇi (Compendium of States or Phenomena). Kessinger Publishing. ISBN 0-7661-4702-9.
- Rhys Davids, T.W. & William Stede (eds.) (1921-5). The Pali Text Society’s Pali–English Dictionary. Chipstead: Pali Text Society. A general on-line search engine for the PED is available at http://dsal.uchicago.edu/dictionaries/pali/.
- Thanissaro Bhikkhu (1996, 1998). Wings to Awakening: An Anthology from the Pali Canon. Retrieved 2007-05-27 from "Access to Insight" at: http://www.accesstoinsight.org/lib/authors/thanissaro/wings/index.html.
- Thanissaro Bhikkhu (trans.) (1997a). Indriya-vibhanga Sutta: Analysis of the Mental Faculties (SN 48.10). Retrieved 2007-05-27 from "Access to Insight" at: http://www.accesstoinsight.org/tipitaka/sn/sn48/sn48.010.than.html.
- Thanissaro Bhikkhu (trans.) (1997b). Sona Sutta: About Sona (AN 6.55). Retrieved 2008-04-15 from "Access to Insight" at http://www.accesstoinsight.org/tipitaka/an/an06/an06.055.than.html.