late 14c., "origin, source, beginning; rule of conduct; axiom, basic assumption; elemental aspect of a craft or discipline," from Anglo-French principle, Old French principe "origin, cause, principle," from Latin principium (plural principia) "a beginning, commencement, origin, first part," in plural "foundation, elements," from princeps (genitive principis) "first man, chief leader; ruler, sovereign," noun use of adjective meaning "that takes first," from primus "first" (see prime (adj.)) + root of capere "to take," from PIE root *kap- "to grasp." Used absolutely for (good or moral) principle from 1650s.
It is often easier to fight for principles than to live up to them. [Adlai Stevenson, speech, New York City, Aug. 27, 1952]
Scientific sense of "general law of nature" is recorded from 1802. The English -l- apparently is by analogy of participle, etc.