Archive image from page 174 of Design for a brain (1954). Design for a brain . designforbrain00ashb Year: 1954 CONSTANCY AND INDEPENDENCE 14/9 of immediate effects shown in Figure 14/8/1. What properties must the three variables B have if the systems A and C are to become independent and absolute ? The question has not only theoretical but practical importance. Many experiments require that one system be shielded from effects coming from others. Thus, a system using magnets may have to be shielded from the effects of the earth's magnetism ; or a thermal system may have to be shielded from the
Archive image from page 174 of Design for a brain (1954). Design for a brain . designforbrain00ashb Year: 1954 CONSTANCY AND INDEPENDENCE 14/9 of immediate effects shown in Figure 14/8/1. What properties must the three variables B have if the systems A and C are to become independent and absolute ? The question has not only theoretical but practical importance. Many experiments require that one system be shielded from effects coming from others. Thus, a system using magnets may have to be shielded from the effects of the earth's magnetism ; or a thermal system may have to be shielded from the effects of changes in the atmospheric tem- Figure 14/8/1. perature ; or the pressure which drives blood through the kidneys may have to be kept independent of changes in the pulse-rate. A first suggestion might be that the three variables B should be removed. But this conceptual removal corresponds to no physical reality : the earth's magnetic field, the atmospheric temperature, the pulse-rate cannot be ' removed '. In fact the answer is capable of proof (S. 24/15) : that A and C should he independent and absolute it is necessary and sufficient tlmt the variables B should be null-functions. In other words, A and C must be separated by a wall of constancies. 14/9. Here are some illustrations to show that the theorem accords with common experience. (a) If A (of Figure 14/8/1) is a system in which heat-changes are being studied, B the temperatures of the parts of the con- tainer, and C the temperatures of the surroundings, then for A to be isolated from C and absolute, it is necessary and sufficient for the B's to be kept constant, (b) Two electrical systems joined by an insulator are independent, if varying slowly, because electrically the insulator is unvarying, (c) The centres in the spinal cord are often made independent of the activities in the brain by a transection of the cord ; but a break in physical con- 159
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