Locke was a sceptic about the possibility of scientific knowledge of corporeal substance. Scientific knowledge is knowledge which is certain, universal, and instructive. According to Locke, to have certain and instructive knowledge of natural kinds (universal knowledge about species of corporeal substances) requires knowledge of the real essence of natural kinds. Since a real essence is the foundation for the properties a thing has, it must be known before a deduction of the properties can be done. Locke did not believe that it was possible for humans to know the real essence of corporeal substances. In my thesis, I provide an explanation for why he held these views. As my work shows, knowledge of the real essence of a natural kind is an involved process that requires first knowing the nominal essence of the natural kind, and then knowing the inner constitution of each member of the kind, knowing which aspect of the inner constitution of each member correlates to the overlap of properties used to delineate the natural kind, and finally, knowing how that aspect, which is the real essence of the natural kind, produces the properties it does. Without knowledge of the mechanics of how the physical real essence produces the mental ideas we cannot know whether the connection between the real essence and the properties is a necessary connection or a mere correlation. Unless we know why there is a connection, we cannot know, with certainty, that the connection will hold in the future or for other like configurations. Locke relies on the mind-body problem to explain why we cannot know the mechanics behind the connections. The mind-body problem has not been given appropriate emphasis in Lockean study. And yet it is uniquely capable of handling the two claims Locke makes about natural science: (1) our knowledge of corporeal bodies can never qualify as scientific knowledge and (2) our knowledge of corporeal bodies can be improved in ways that are useful to human life.