This logic, applied to the human organism, says that if we can know what is abnormal then we can know what is normal. It is upon this thinking that the practice of medicine is based. The M.D. is able to determine that a bodily function is abnormal. For example, when the blood pressure reaches a certain point, a CVA may occur, or other physiological functions take place that may lead to death. From this the physician concludes that at a certain point the blood pressure is considered to be abnormal.
Hegelian logic teaches that for every thesis there is an antithesis.
If there is a level that is abnormal, then there must be a level that is normal. That is logical. However, the physician makes his error in concluding that if he is able to ascertain what is abnormal, then he is also able to determine what is normal. Here is where the logic breaks down and for a very simple reason. You cannot use the same reasoning for the thesis and the antithesis.