Origin and history of declaration

c. 1300, cler, "giving light, shining, luminous;" also "not turbid; transparent, allowing light to pass through; free from impurities; morally pure, guiltless, innocent;" from Old French cler "clear" (of sight and hearing), "light, bright, shining; sparse" (12c., Modern French clair), from Latin clarus "clear, loud," of sounds; figuratively "manifest, plain, evident," in transferred use, of sights, "bright, distinct;" also "illustrious, famous, glorious" (source of Italian chiaro, Spanish claro), which is reconstructed to be from PIE *kle-ro-, from root *kele- (2) "to shout."

The reconstructed prehistoric sense evolution to light and color involves an identification of the spreading of sound and the spreading of light (compare English loud, used of colors; German hell "clear, bright, shining," of pitch, "distinct, ringing, high").

Also from c. 1300 in English "beautiful, magnificent, excellent;" of land, "cleared, leveled." of colors, "bright, pure;" of weather or the sky or sea, "not stormy; mild, fair, not overcast, fully light, free from darkness or clouds;" of the eyes or vision, "clear, keen;" of the voice or sound, "plainly audible, distinct, resonant."

Also of the mind, "keen-witted, perspicacious;" of words or speech, "readily understood, manifest to the mind, lucid" (an Old English word for this was sweotol "distinct, clear, evident"); of land, "cleared, leveled." 

By c. 1300 in reference to complexions. Of possession or title, "unrestricted, unconditional, absolute," attested by early 15c. 

The sense of "free from encumbrance," later largely nautical, developed c. 1500. The meaning "obvious to the senses" is from 1835. 

Clear-sighted is from 1580s (clear-eyed is from 1520s); clear-headed is from 1709. For coast is clear see clear (v.).

active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.

As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.