 The thesis that there is a fundamental difference between man-made products and natural substances has had a long-lasting influence. In the Middle Ages, Avicenna criticized alchemy on the ground that it can never produce ‘genuine’ substances (Briffault 1930: 147). Even today, some still maintain that there is a difference between, for example, natural and synthetic vitamin C. The modern discussion of this theme is taken up in  Section 2.5.