We reject theories for being bad explanations (of reality), and accept theories for being good ones. How do we know which are which?
The following properties make theories better:
- says more (deeper)
- explains what it purports to
- bold (exposes itself to refutation by all sorts of observations)
- supported by good arguments
The following properties make theories worse:
- contains unexplained complications
- is not consistent with some observation
- criticised by good arguments
Note the use of comparative words. There is no way to measure how good a theory is in absolute terms, only compared to its rivals.
I probably left out some important things, because I tend to do this very intuitively. Please comment on any glaring omission. (And yes I'm aware some items are a bit redundant -- redundancy doesn't hurt anything and can help.)