  Du Bos held that scientific investigation always involves some uncertainty. He wrote that “I distrust physical explanations, since the imperfection of this science almost always makes guesswork necessary” (2.2). According to Du Bos, a scientific hypothesis is only probable to some extent or another. He illustrates this view by reference to Harvey’s hypothesis concerning the circulation of blood. Du Bos writes that, “Most scholars of his [Harvey’s] time were persuaded by his views. It was as firmly and widely established as a scientific truth as something that is not directly observed can be. That is, it was regarded as a view more probable than the contrary opinion” (2.33).