theory (n.)
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.