(4) During the course of the twentieth century received scientific opinion became even more restrictive about possible causes of physical effects, and ruled out any sui generis mental or vital causes, even of a law-governed and predictable kind. Detailed physiological research, especially into the operation of nerve cells, gave no indication of any physical effects that cannot be explained in terms of basic physical forces that also occur outside living bodies. By the middle of the twentieth century, belief in sui generis mental or vital forces had become a minority view. This led to the widespread acceptance of the doctrine now known as the “causal closure” or the “causal completeness of the physical”, according to which all physical effects have fully physical causes.