A rare gem for the philosophically inclined but physically minded. As may be seen from his other books, Henry Margenau has a gift for explaining the physical significance of mathematical physics. Bringing philosophical constructs into the foray is no mean task, and one can argue, at least in academia today, that physics progresses fine without the philosophical baggage. This is a delusion, as one will realise when treating the concepts at the heart of physics, such as the nature of reality, with due diligence. Margenau's book leads the way. It will be of help to anyone studying the natural sciences and the philosophy of sciences.