It's an extension of Occam's razor which says "All other things being equal, the simplest solution is the best."
But overlays the principle with scientific scepticism.
Always question the solution because things are not normally that simple.
Occam is right for most practical purposes, but Whitehead is more right at a theoretical level.
A good example is atomic theory. For most practical purposes treating the atom as the smallest particle of matter (as most scientist thought it was, for many years) is simple and works very well. However when you look more deeply the atom turns out to be a very complicated structure indeed and not at all simple.
