On Scientific Theories

A scientific theory must 'save' or 'preserve' the appearances, the phenomena, it deals with, in the sense of getting them all in, doing justice to them. Thus for example, your phenomena are luminous points in the night sky which exhibit such and such movement in relation to one another and in relation to an observer at a particular point, or various chosen points, on the surface of the Earth. Your astronomical theory will be a supposal such that, if it were true, the apparent motions from the point or points of observation would be those you have actually observed. The theory will then have 'got in' or 'saved' the appearances.

But if we demanded no more than that from a theory, science would be impossible, for a lively inventive faculty could devise a good many different supposals which would equally save the phenomena. We have therefore had to supplement the canon of saving the phenomena by another canon--first, perhaps, formulated with full clarity by Occam. According to this second canon we must accept (provisionally) not any theory which saves the phenomena but that theory which does so with the fewest possible assumptions. Thus the two theories (a) that the bad bits in Shakespeare were all put in by adapters, and (b) that Shakespeare wrote them when he was not at his best, will equally 'save' the appearances. But we already know that there was such a person as Shakespeare and that writers are not always at their best. If scholarship hopes ever to achieve the steady progress of the sciences, we must therefore (provisionally) accept the second theory. If we can explain the bad bits without the assumption of an adapter, we must.

In every age it will be apparent to accurate thinkers that scientific theories, being arrived at in the way I have described, are never statement of fact. That stars appear to move in such and such ways, or that substances behaved thus and thus in the laboratory; these are statements of fact. The astronomical or chemical theory can never be more than provisional. It will have to be abandoned if a more ingenious person thinks of a supposal which would 'save' the observed phenomena with still fewer assumptions, or if we discover new phenomena which it cannot save at all.
~C.S. Lewis, The Discarded Image: An Introduction to Medieval and Renaissance Literature, "Reservations" (1964)