theoryyoudaoicibaDictYouDict[theory 词源字典]
theory: [16] The etymological notion underlying theory is of ‘looking’; only secondarily did it develop via ‘contemplation’ to ‘mental conception’. It comes via late Latin theōria from Greek theōríā ‘contemplation, speculation, theory’. This was a derivative of theōrós ‘spectator’, which was formed from the base thea- (source also of theásthai ‘watch, look at’, from which English gets theatre).

Also derived from theōrós was theōreín ‘look at’, which formed the basis of theórēma ‘speculation, intuition, theory’, acquired by English via late Latin theōrēma as theorem [16]. From the same source comes theoretical [17].
=> theatre[theory etymology, theory origin, 英语词源]
theory (n.)youdaoicibaDictYouDict
1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theoria "contemplation, speculation; a looking at, viewing; a sight, show, spectacle, things looked at," from theorein "to consider, speculate, look at," from theoros "spectator," from thea "a view" (see theater) + horan "to see," possibly from PIE root *wer- (4) "to perceive" (see ward (n.)).

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.