Almost everything in metaphysics is controversial, and it is therefore not surprising that there is little agreement among those who call themselves metaphysicians about what precisely it is that they are attempting. In beginning a discussion of the nature and validation of metaphysical arguments and theories, the best course we can follow is to list some of the standing preoccupations and ambitions of metaphysicians. For this purpose we need to make the assumption that there is a distinct class of metaphysical philosophers, a class into which such thinkers as Plato, Thomas Aquinas, René Descartes, Benedict de Spinoza, and G. W. F. Hegel would fall and from which purely critical or analytic philosophers like the later G. E. Moore would be excluded. It has to be admitted, however, that the line between metaphysical and nonmetaphysical philosophy is exceedingly hard to draw, for many metaphysicians from...