This work speaks about very special solution of the mind–body problem. This solution based on the so-called Principle of Co-existence stands out as one of the most interesting attempts at solving the mind–body problem. It states that substances can only exert a mutual influence on one another if they have something in common. This does not have to be a common property but rather, a binding relationship. Thus, substances co-exist when they remain bound by a common relationship, for instance, to an external subject. The Principle of Co-existence played an extremely important role in Kant's philosophy, not only since it provided a framework for solving the mind–body problem, but since it captured the very basis of its existence. The Principle found also reflection in the works of Kant's successors, such as Fichte, Schelling, Hegel or Feuerbach. It had significant—though often hidden—repercussions on later philosophy of mind. The notion of force and the principle of its operation became key concepts in resolving the mind–body problem. As a result, philosophy of mind concentrated on the search for a principle explaining the occurrence of two complementary types of phenomena. This established a tradition which, to a greater or lesser extent, has survived to our day.