word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."
In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
"of or pertaining to the act or power of perceiving," 1650s, from Latin percept-, past-participle stem of percipere (see perceive) + -ive. In reference to intelligence from 1860. From mid-15c. as the name of a type of optical instrument or magic glass revealing future events. The older word in the mental sense was perceptible (q.v.); also compare Middle English perceivaunt "observant" (late 14c.), from Old French and Medieval Latin. Related: Perceptively; perceptiveness.