[This four-part series on the nature of theory is based on a reading of C.S. Lewis' The Discarded Image. An Introduction to Medieval and Renaissance Literature.]
“The great masters do not take any model, or any theory, quite so seriously as the rest of us. They know that it is, after all, only a model, only a theory, and as such possibly replaceable.”
What then is a theory?
First principle
The business of a natural philosopher, or scientist, is to construct theories which will ’save appearances’. A scientific theory, according to Aristotle [De Caelo], 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 movements 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 points of observation would be those you have actually observed. The theory will then have ‘got in’ or ’saved’ the appearances.
Second principle
But if we demanded no more than that from a theory, science would be impossible, for a lively inventive intellect could devise a good many different supposals which would equally save the phenomena. We therefore have to supplement the first principle of saving the phenomena by another principle - first, perhaps, formulated with full clarity by Occam. According to this second principle we must accept (provisionally) not any theory which saves the phenomena but that theory which does so with the fewest possible assumptions.
Thus, for example, 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 literary 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.
To conclude
In every age it will be apparent to accurate thinkers that scientific theories, being arrived at in the way I have described, are never statements 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.
This would, I believe, be recognised by all thoughtful scientists today. It was recognized, for instance, by Newton if, as I am told, he wrote not ‘the attraction varies inversely as the square of the distance’, but ‘all happens as if’ it so varied. It was certainly recognised in the Middle Ages. ‘In astronomy’, says Aquinas, ‘an account is given of eccentrics and epicycles on the ground that if their assumption is made the sensible appearances as regards celestial motions can be saved. But this is not a strict proof since for all we know they could also be saved by some different assumption’.
The real reason why Copernicus raised no ripple and Galileo raised a storm, may well be that whereas the one offered a new supposal about celestial motions, the other insisted on treating this supposal as fact. If so, the real revolution, as Owen Barfield writes, consisted not in a new theory of the heavens but in ‘a new theory of the nature of theory’.