1818, as a term in church administration, "the holding by one person of two or more offices at the same time," from plural + -ism. Attested from 1882 as a term in philosophy for a theory which recognizes more than one ultimate principle. In political science, attested from 1919 (in Harold J. Laski) in the sense of "theory which opposes monolithic state power." General sense of "toleration of diversity within a society or state" is from 1933. Related: Pluralist (1620s, in the church sense); pluralistic.
"the theory that the mental powers of the individual consist of independent faculties, each of which has its seat in a different brain region, whose size is commensurate with the power of the faculty," 1815, literally "mental science," from phreno- "mind" (q.v.) + -logy "study of." Applied to the theory of mental faculties originated by Gall and Spurzheim that led to the 1840s mania for reading personality clues in the shape of a subject's skull and the "bumps" of the head. It was most popular from about 1810 to 1840. Related: Phrenological; phrenologist.
This theory, which originated at the close of the eighteenth century, assumes, moreover, as an essential part, the plasticity of the cranial envelop, by which the skull conforms externally, in the normal subject, to the shape and configuration of the brain within, so that its form and faculties may be determined, with sufficient exactness, from the skull itself, whether in the skeleton or in the living person. [Century Dictionary]
mid-14c., "a plotting of evil, unlawful design; a combination of persons for an evil purpose," from Anglo-French conspiracie, Old French conspiracie "conspiracy, plot," from Latin conspirationem (nominative conspiratio) "agreement, union, unanimity," noun of action from past-participle stem of conspirare "to agree, unite, plot," literally "to breathe together" (see conspire).
Earlier in same sense was conspiration (early 14c.), from French conspiration (13c.), from Latin conspirationem. An Old English word for it was facengecwis.
Conspiracy theory "explanation of an event or situation involving unwarranted belief that it is caused by a conspiracy among powerful forces" emerged in mid-20c. (by 1937) and figures in the writings of, or about, Charles Beard, Hofstadter, Veblen, etc., but the degree of paranoia and unreasonableness implied in each use is not always easy to discern. The phrase was used from 19c. in a non-pejorative sense "the theory that a (certain) conspiracy exists," especially in court cases. Its use in general reference to theories of hidden cabals pulling wires behind the scenes of national or global events is by 1871.
We shall better understand the ensuing civil war if we study the movements in the four most important of these States, in relation to a theory which asserts that the secession was a conspiracy whose central cabal, composed of Southern senators and representatives in Washington, dictated through its ramifications in the States the inception and the course of the revolution. [James Ford Rhodes, page headed "The Conspiracy Theory" in "History of the United States from the Compromise of 1850," New York, 1893]
To the Jingo Imperialist "the South African Conspiracy" is the alleged Dutch conspiracy to drive the British into the sea. But, to the man accustomed to weigh evidence and to base his opinions on ascertained facts, it is clear that this conspiracy theory is absolutely untenable, for whatever "evidence" has been adduced in support of the theory is nebulous and shadowy in the extreme. ["The South African Conspiracy," in The Westminster Review, January 1902]
"relating to or of the nature of a flood," 1650s, from Latin diluvium "flood, inundation," from diluere "wash away," from dis- "away" (see dis-) + -luere, combining form of lavere "to wash" (from PIE root *leue- "to wash"). Related: Diluvianism (1816) "geological theory supposing the occurrence of a former universal deluge."
c. 1500, santren "to muse, be in reverie," a word of uncertain origin. The meaning "walk with a leisurely gait" is from 1660s, and may be a different word which, despite many absurd speculations, also is of unknown origin. Klein prints the theory (held by Skeat and Murray) that this sense of the word derives via Anglo-French sauntrer (mid-14c.) from French s'aventurer "to take risks." Century Dictionary finds the theory involves difficulties but "it is the only one that has any plausibility," but OED finds it "unlikely." Also see here. Related: Sauntered; saunterer; sauntering.