Thesis #1 The biological fitness of a population is the average net reproduction of its members, which in turn is determined by their capacity to survive and reproduce; biological fitness is at the core of health.
The starting point of the present theses is to take Darwinian fitness as the key foundation for understanding health. Stripped of such a Darwinian foundation, a wide spectrum of alternative definitions of health can be offered as the central focus for medicine: biochemical efficiency, athletic performance, freedom from disease, and so on. None of these definitions have particularly well-formulated scientific foundations, and certainly none of them connect to a scientific theory with the power, range, and depth of evolutionary theory. It would be interesting to provide an historically well-developed presentation of the many alternative conceptions of health that have been promulgated and debated over the last few thousand years of Western medicine, leaving aside the more exotic traditions of traditional Eastern medicine or the shamanistic practices of pre-agricultural societies. But instead, I will just get on with the task of developing of a coherent, scientifically formulated, alternative to the various confusions that are widely on offer.
Darwinian fitness is a single numerical measure which combines all the individual probabilities of survival and reproduction of a population of individuals in the particular environment in which it finds itself. The mathematical form of this measure takes different forms with different population demographies, but it can always be defined in a scientifically cogent manner.
So why is Darwinian fitness a useful place to start developing the scientific foundations of medicine? In all the biological sciences, there are few other variables for which we have such well-developed theory. Moreover, it is Darwinian fitness that is the key to the arc of evolution. This Darwinian variety of fitness is the key determinant of natural selection, and natural selection is the steering wheel for evolution.
But natural selection is not an all-powerful determinant of evolutionary processes. Instead, it is constrained, limited, and often thwarted by other evolutionary factors. Such constraints are the main theme of the first ten theses. It is not possible to think about natural selection intelligently unless the constraints on its action are kept firmly in mind. In particular, it is not a synonym for a benign, all-powerful, cosmic force that always maximizes our Darwinian fitness.