How do we know what goes on inside the body? Medical science proposes ways to know. While diagnostic tests have a scientific basis, however, their evaluation often depends on judgment calls and statistical probability. The only way to have true certainty is to actually create either the condition that is desired or the condition that is feared. This follows from a general principle whose implications are little recognized: that the only certain knowledge is of humanly defined systems. This is why the truths of mathematics are abiding while scientific theories about nature are provisional. I suspect it is the basis of religious faith. It is certainly the basis of digital computation, where a state is simply yes or no.

Western medicine does not pretend to create a state of health, which could never be absolutely verified in any case. The problem of induction is that one cannot prove generalizations like “all crows are black” (whereas it only takes one crow of another color to disprove the claim). Similarly, if health cannot be verified, it can at least be disproven. This is the inevitable logic of diagnosis, which does not look for health but for disease. If you undergo a biopsy, for example, a “clean bill of health” is not a realistic outcome to expect. If the test fails to indicate suspicious tissue, it may be repeated until it does, or lead to other tests. The way to diagnostic certainty, therefore, is to find the disease that is feared. The outcome that best satisfies diagnosis is bad news, because it leads to further action. The diagnostic interventions themselves may be harmful, and the procedures to follow have their own risks.

The body is a “black box,” in the sense that, without disturbing its integrity, we observe only what goes into it and what comes out. Medical science proposes to observe what goes on inside as well, through x-rays, magnetic resonance images, biopsy, laparoscopy, or literally opening it in surgery. It is worthwhile to distinguish these technologies, for they may vary greatly in their effects upon the state of the tissues investigated. A varying price is paid for the view.

Knowledge is never free of consequence, and never free of interpretation. Modern x-rays may be relatively harmless, but they are not quite the same as direct inspection in ordinary light. Doctors may feel around inside you or look around with some device, which may entail little risk to your health. The consequences of biopsy, on the other hand, could be riskier. What the doctor recommends will be a matter of interpretation and guesswork as well as of what is objectively there. Your prognosis, as well as your diagnosis, may rely on statistics—that is, on average outcomes for other people. Those are probabilities, not certainties. Yet, the patient must come to a definite decision. Even with expert opinion, we hold our lives in our own hands.