The earliest discovery of Pasteur, and for him the most exciting in all his life, was the asymmetry of molecules as a specific characteristic of living organisms-in other words, the fact that the molecules of living matter come in two varieties which, though chemically identical, are in their spatial structure like mirror images to each other-or like right and left gloves. 'Left-handed' molecules rotate polarized light to the left, 'right-handed' molecules to the right; life substances are thus 'optically active'. Why this should be so we still do not quite know; but it remains a challenging fact that 'no other chemical characteristic is as distinctive of living organisms as is optical activity'.