Chapters: The Physical Meaning of Poincaré Massless Representations, Massless Representations, Massless Fields are Different, How to Couple Massless and Massive Matter, The Behaviour of Matter in Fields, Geometrical Reasons for the Poincaré Group, Description of the Electromagnetic Field, The Equations Governing Free Gravitation, How Matter Determines Gravitational Fields, Nonlinearity and Geometry, Quantum Gravity.