9.1 Variation and Uncertainty
Variation often gives rise to uncertainty. Though we can recognize a group of objects called teapots, the variation present from one teapot to another results in our being uncertain whether we shall spill some of the tea when first pouring from a strange pot, for some pots are good pourers, some are not. More seriously, all biological material exhibits variation; even the simple influenza virus varies, with the consequence that we are uncertain what vaccine to use against it. Human beings show variation that we rightly cherish, yet it gives rise to uncertainty, whether in what size of trousers a retailer should stock, or in a stranger's reaction to a request.
There are only a few topics where variation is not present. Precision engineering is capable of making objects, like the balls in the urn, that are, for practical purposes, indistinguishable and portray no obvious variation. One atom of an isotope of hydrogen is regarded as the same as any other atom and the behavior of the isotope in the presence of oxygen can be predicted perfectly. We can say, in the spirit of §7.3, that one atom can be exchanged for another. Physics and chemistry are both founded on this lack of variation that partly explains why physical scientists were so uncomfortable with quantum physics and its unpredictability. It also helps to explain why those two subjects have advanced more than others, like biology, because they are not hampered by variation ...