理屈 and 理論: Logic, excuses, and a theory's support
Compare the logic of a mechanism and plausible excuses with a theory examined through experiment or still lacking evidence. Identify an explanation's role without making the words guarantee truth or falsehood.
Logic of a structure and plausible excuses
理屈 includes logic and excuse. Understanding how a structure works and offering plausible-sounding excuses are different uses of the same word.
「仕組みの理屈が分かれば、故障の原因も見つけやすい。」 (Understanding how the structure works makes it easier to find the cause of a malfunction.) Understanding the structure makes the cause of a malfunction easier to find. The sentence does not report that the cause has already been found.
Excuses and a theory's evidence
A separate example reports that a late arrival began offering plausible excuses. That context realizes the excuse sense without defining every use of 理屈 as a lie or excuse.
「彼の理論は興味深いものの、まだ十分な証拠がありません。」 (His theory is interesting, but it still lacks sufficient evidence.) The theory is interesting but still lacks sufficient evidence. Its name does not guarantee completed proof, and insufficient support is not itself a final finding of error.
Read the role and the stated support
Another 理論 example reports confirming a theory learned in a lecture through a simple experiment. That completed check and the example of insufficient evidence belong to separate sentences, not a chronology of one theory.
To read 理屈, inspect whether the context concerns a structure's logic or excuses. For 理論, inspect the stated check or evidence without using the labels to divide every explanation into false versus true.
Must 仕組みの理屈 and the 理屈 offered after arriving late be read in only one sense?
Show the answer and why
No. The structure example relates understanding its logic to easier investigation of a cause; the late-arrival example reports beginning to offer plausible excuses. A theory's experimental check or insufficient evidence also stays within its own sentence, rather than letting the word alone establish truth or falsehood.