petitio principii

mid-14c., petiocioun, "a supplication or prayer," especially to a deity," from Anglo-French (early 14c.), from Old French peticion "request, petition" (12c., Modern French pétition) and directly from Latin petitionem (nominative petitio) "a blow, thrust, attack, aim; a seeking, searching," in law "a claim, suit," noun of action from past-participle stem of petere "to make for, go to; attack, assail; seek, strive after; ask for, beg, beseech, request; fetch; derive; demand, require," from PIE root *pet- "to rush; to fly."

Meaning "formal written request to a superior (earthly)" is attested from early 15c. In law, "a written application for an order of the court" (1737).

late 14c., "origin, source, beginning" (a sense now obsolete), also "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").

 The English -l- apparently is by analogy of participle, manciple, etc., also principal. From the notion of "one of the fundamental tenets or doctrines of a system, a law or truth on which others are founded" comes the sense of "a right rule of conduct" (1530s).

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," by virtue of which a machine or instrument operates, is recorded from 1802.