Question

In 1907, Dr. Duncan MacDougall performed a series of experiments in which he attempted to measure the weight of the soul as it left a dying person. In his experiments, MacDougall placed a dying person on a scale and measured their weight immediately prior to and following death. MacDougall determined the change in weight to be approximately 21 grams. From these experiments, he concluded that the soul exists and has mass. What is the fundamental scientific flaw in his conclusions?