The Japanese language has two separate words equivalent to the two usages of the English word, 'theory'.
A system of knowledge built on logic to systematically and uniformly explain individual phenomena. In addition, a purely logical knowledge corresponding to practice.
1. A principle (belief) or claim for certain things.
I tried to translate the Japanese definitions as faithfully as I can into English. The first word, 'ri-ron', is equivalent to the scientific usage of the English word, 'theory' (as in 'the theory of evolution' or 'the theory of relativity'), while the second word, 'se-tsu', is equivalent to the common usage (more like 'I have a theory!'). Setsu doesn't need to be substantiated and can be wild claims.
It's a shame the English language doesn't have two separate words; we can avoid a lot of confusion.