Roderick Chisholm has been for many years one of the most important and influential philosophers contributing to metaphysics, philosophy of mind, and epistemology. This book can be viewed as a summation of his views on an enormous range of topics in metaphysics and epistemology. Yet it is written in the terse, lucid, unpretentious style that has become a hallmark of Chisholm's work. The book is an original treatise designed to defend an original, non-Aristotelian theory of categories. Chisholm argues that there are necessary things and contingent things; necessary things being things that are not capable of coming into being or passing away. He defends the argument from design, and thus includes the category of necessary substance (God). Further contentions of the essay are that attributes are also necessary beings, but not necessary substances, and that human beings are contingent substances but may not be material substances.