"Linearity" is the (unfortunate) jargon term for proper progression. Building a house normally starts from the foundation and progresses upwards. Linearity gains its respectability from science — by mistake. Progress in science is linear only locally: theories of gravity appear in succession, each built on its immediate predecessor. But different branches of science are not linear. A scientific theory unifies some theories and some observations, and gives rise to others. This too is not linear — it cannot possibly be.