Theory
late 16th century (denoting a mental scheme of something to be done): via late Latin from Greek theōria ‘contemplation, speculation’, from theōros ‘spectator’.
wiktionary
From Middle French théorie, from Late Latin theōria, from Ancient Greek θεωρία(theōría, “contemplation, speculation, a looking at, things looked at”), from θεωρέω(theōréō, “I look at, view, consider, examine”), from θεωρός(theōrós, “spectator”), from θέα(théa, “view”) + ὁράω(horáō, “I see, look”) [i. e. θέαν ὁράω (théan horáō, “see, look at a view; survey + genitive”)].
etymonline
theory (n.)
1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theōria "contemplation, speculation; a looking at, viewing; a sight, show, spectacle, things looked at," from theōrein "to consider, speculate, look at," from theōros "spectator," from thea "a view" (see theater) + horan "to see," which is possibly from PIE root *wer- (3) "to perceive."
Earlier in this sense was theorical (n.), late 15c. Sense of "principles or methods of a science or art" (rather than its practice) is first recorded 1610s (as in music theory, which is the science of musical composition, apart from practice or performance). Sense of "an intelligible explanation based on observation and reasoning" is from 1630s.
