Natural Philosophy is based on a current understanding of Physical Reality. Thus it must evolve as our knowledge of Nature changes. This was true in Ancient Greece. It remains true up to the present. We can classify views of Physical Reality into four periods that correspond to major periods in Natural Philosophy: Aristotelian Newtonian Atomic Octonion Cosmological This book describes Natural Philosophy in view of the exciting new developments of Physical Reality in Octonion Cosmology. It begins with a detailed narrative outline of the genesis of the ten spaces (universes) of the Cosmos from a point in a 20 dimension space-time. It is followed by a detailed view of the Octonion Cosmological Period describing the impact on Epistemology and Metaphysics. Space, time and matter are intertwined in a new manner that differs from the Atomic Period. The structure of the Cosmos differs dramatically from previous theories. The book also addresses Predestination, Fate, and Free Will from a quantum perspective. Its third part contains the original book: 21st Century Natural Philosophy, which describes the Natural Philosophy of the Atomic Period. The Atomic Period began in the late 19th Century when the atomic nature of matter was finally accepted. It continued up to the 2020 as atoms were disassembled into elementary particles and forces. In January, 2020 the author introduced a hypercomplex number based theory, QUeST, that had the remarkable feature of yielding the known interactions and elementary particles of The Standard Model and much more. Subsequently Octonion Cosmology emerged. It is described in 2021 in the author's books: Quantum Space Theory and Beyond Octonion Cosmology. A point of interest is a "golden" triangle of transformations between mass, energy, and space. Mass can be transformed into energy and vice versa. Mass and/or energy can be transformed into space and vice versa.