---------------------------------------------- S16d. What, then, distinguishes a good theory? ---------------------------------------------- We can _know_ whether a theory has been correct in the past, and we can _trust_ that it will remain so in the future. There is no other kind of knowledge than that of the past. Relying on that ''anything in the future is like in the past'' is an act of faith. The question is not about faith or not, but about faith in what is best supported by past experience. Theories that conform with the past are easy to trust. But they come in different degrees of stringency. Theories which are not restrictive at all but accommodates everything (such as astrology or psychoanalysis) are in vogue (as society shows) but useless (and probably harmful). These are the ones that Popper calls unfalsifiable. Highly restrictive theories (what Popper calls scientific) are preferred by those who want to control their destiny as far as possible. Theories like Newton's, general relativity, or QED are extremely restrictive and in agreement with past experience, hence both trustworthy and very useful. What makes a theory good is not its potential falsifiability, but that it drastically reduces the number of possibilities which are present without the theory, without eliminating something that can actually happen. If you have no theory and put two marbles into your empty pocket, and then another two, you don't know how many marbles you can take out. If you know arithmetic and the law of conservation of marbles you can predict that exactly four can be taken out. This is testable, and will always come out correct. So you have a correct theory. Of course, its validity is not unlimited, since it assumes that your pocket does not have a hole; so if some experiment does not conform to your theory since you can only take out three, you suspect that the domain of validity was violated; you check for the hole - and surely you'll find it. This is exactly analogous to the way Newton's theory works, within its domain of validity. If it fails, we suspect speed close to c, or highly accurate measurements, or tiny distances. And surely we'll find it so.