"principle," properly "a thing held (to be true)," early 15c., from Latin tenet "he holds," third person singular present indicative of tenere "to hold, to keep, to maintain" from PIE root *ten- "to stretch" (cf. Sanskrit tantram "loom," tanoti "stretches, lasts;" Persian tar "string;" Lithuanian tankus "compact," i.e. "tightened;" Greek teinein "to stretch," tasis "a stretching, tension," tenos "sinew," tetanos "stiff, rigid," tonos "string," hence "sound, pitch;" Latin tendere "to stretch," tenuis "thin, rare, fine;" Old Church Slavonic tento "cord;" Old English thynne "thin"). Connection notion between "stretch" and "hold" is "to cause to maintain." The modern sense is probably because tenet was used in Medieval Latin to introduce a statement of doctrine.