“Mendicants, there are these five faculties. What five? The faculties of faith, energy, mindfulness, immersion, and wisdom. These are the five faculties.”
Indriya (“faculty”) is an abstract noun from indra, the name of the potent Vedic god of war. In the Vedas, Indra manifests his indriya by drinking soma. The drug enables him to release his full potential and power; originally this probably referred to drinking an amphetamine-like substance before battle. Thus indriya means “faculty” in the sense “capability”, but also has the sense “potential, potency”.
This Saṁyutta is unique in that it gathers several different kinds of things under the banner of indriya. The five spiritual faculties predominate, but we also find the sense faculties, usually reckoned as six (SN 48.25–30), but only five in SN 48.41 and SN 48.42; the five faculties of feeling (SN 48.31–40); the three faculties pertaining to life (SN 48.22); the three faculties pertaining to enlightenment (SN 48.23. Together these were systematized as the Abhidhamma set of 22 faculties (Vb 5).
These are the five so-called “spiritual faculties”, whose specification is identical to the “five powers” (SN 50.1–12). For definitions, see SN 48.8, SN 48.9, and SN 48.10.