a. [ L. facultas, -atis, faculty: cf. F. facultatif, G. fakultativ. ] 1. Having relation to the grant or exercise faculty, or authority, privilege, license, or the like hence, optional; as, facultative enactments, or those which convey a faculty, or permission; the facultative referendum of Switzerland is one that is optional with the people and is necessary only when demanded by petition; facultative studies; -- opposed to obligatory and compulsory, and sometimes used with to. [ Webster 1913 Suppl. ]

2. Of such a character as to admit of existing under various forms or conditions, or of happening or not happening, or the like; specif.: (Biol.) Having the power to live under different conditions; as, a facultative parasite, a plant which is normally saprophytic, but which may exist wholly or in part as a parasite; -- opposed to obligate. [ Webster 1913 Suppl. ]

3. (Physiol.) Pertaining to a faculty or faculties.

In short, there is no facultative plurality in the mind; it is a single organ of true judgment for all purposes, cognitive or practical. J. Martineau. [ Webster 1913 Suppl. ]