Etymology
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logical (adj.)
early 15c., "based on reason, according to the principles of logic," from logic + -al (1). Meaning "pertaining to logic" is c. 1500. Attested from 1860 as "following as a reasonable consequence." Related: Logically. Logical positivism, in reference to the ideas of the Vienna Circle of philosophers, is from 1931.
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leading (adj.)
1590s, "that goes first," present-participle adjective from lead (v.1). Meaning "directing, guiding" is from 1620s. Of persons, "having first or most prominent place," 1670s. In reference to theatrical companies, leading lady is from 1846; leading man from 1847.
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principle (n.)

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.

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leading (n.2)
mid-13c., "a bringing by force," verbal noun from lead (v.1). Meaning "direction, guidance" is from late 14c.
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leading (n.1)
"lead work; lead covering or frame of lead," mid-15c., verbal noun from lead (n.1). Printing sense is from 1855.
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Peter Principle (n.)

1968, "in a hierarchy, every employee tends to rise to his level of incompetence," named for (and by) Laurence Johnston Peter (1919-1990) Canadian-born U.S. educationalist and author, who described it in his book of the same name (1969).

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Bernoulli's principle 
named for Dutch mathematician Daniel Bernoulli (1700-1782), who published it in 1738. The family produced several noted mathematicians.
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apagoge (n.)
"demonstration of a proposition by the refutation of its opposite, indirect proof, reductio ad absurdum," from Greek apagoge "a leading away" (used by Aristotle in a logical sense), from apagein "to lead away," from assimilated form of apo "from, away from" (see apo-) + agein "push forward, put in motion; stir up; excite, urge," from PIE root *ag- "to drive, draw out or forth, move." Related: Apogogic (1670s); apogogical.
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keynote (n.)
also key-note, "lowest note of a musical scale, basis of a tonal key, the tonic," 1776, from key (n.1) in sense of "musical scale" + note (n.). The keynote is the given note on which the melodic and harmonic relationships in the scale are built, and it gives its name to the key. Figurative sense of "leading idea, central principle" is from 1783; keynote speech is 1887, American English.
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conclusive (adj.)

1610s, "occurring at the end," from French conclusif, from Late Latin conclusivus, from conclus-, past participle stem of Latin concludere "to shut up, enclose," from assimilated form of com "together" (see con-) + -cludere, combining form of claudere "to shut" (see close (v.)). Meaning "definitive, decisive, convincing, being so forcible as not to admit of contradiction" (on the notion of "leading to a logical conclusion," and thus putting an end to debate) is from 1640s. Related: Conclusiveness.

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