MacDougall at least had a crack at proving its existence In 1901, one of the most famous metaphysical experiments of the 20th century was performed by a Massachusetts physician. His name was Duncan MacDougall, and he believed that, if the soul were real, it should have measurable weight. He therefore attempted to compare the weights of patients before and after death. After testing six patients dying of tuberculosis, he concluded that dying results in the small but measurable loss of ¾ of an ounce – the weight of the soul. To MacDougall’s way of thinking, the soul should be found only in human beings and not in other animals. He therefore performed similar measurements in dogs and found no loss in weight as the animals expired. This he regarded as confirmation of his belief that souls are found only in living human beings, and that when a human being dies, the soul leaves the body. MacDougall’s investigations were flawed at many levels. When his results were first published, critics argued that the weight loss could be explained by physiologic factors, such as evaporation. Moreover, his report failed to mention several patients in whom he found no weight loss. Finally, subsequent attempts to reproduce his results failed to find any weight loss. Indeed, MacDougall’s vision may have been clouded by confirmation bias, the tendency for investigators to see what they expect.