An earlier version of this paper has been read at a conference on Mental Causation which was held on March 12–14, 1990, at the Center for Interdisciplinary Research (ZiF), University of Bielefeld, as an integral part of the work of the research group Mind and Brain. I am very grateful to ZiF for the financial support that made it possible for me to take part in the research group. Thanks also to the organizers of the conference Peter Bieri and Jaegwon Kim.