What does theory mean here? First, theory signals 'speculation. But a theory is not the same as a guess. My guess is that would suggest that there is a right answer, which I don't happen to know: My guess is that Laura just got tired of Michael's carping, but we'll find out for sure when their friend Mary gets here. A theory, by contrast. is speculation that might not be affected by what Mary says, an explanation whose truth or falsity might be hard to demonstrate.