This book develops a 21st century Metaphysics and Natural Philosophy based on the Physics of Quantum Theory, and Special and General Relativity. It suggests that modern Physics reduces Metaphysics to a much simpler form than the Metaphysical speculations of recent centuries. It returns to the original monistic ideas of Thales (and his successors) and shows that Nature at the sub-quantum level leads directly to the view that the universe is composed of one substance that can assume a variety of forms that we call elementary particles and can also have a "lack" of form (by being an infinite superposition of all forms) that we call the vacuum. The Natural Philosophy presented in this volume is based on the general nature of quantum field theory, and particularly on modern Elementary Particle theory.Among items of interest is a precise modern view of Causation and Change, a proof that all physical Reality is contingent, the introduction of a new Modality based on the concept of contingently necessary, an explanation of the nature of time and space, a demonstration that The Standard Model of Elementary Particles is ultimately based on Logic, as well as many other points differentiating 21st century Metaphysics from classical Metaphysics. In the interests of reaching a wide audience, there is no mathematics in this work. Those wishing to see mathematical detail are directed to earlier work by this author.