Sachs M. General Relativity and Matter...1982

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There exist essentially two levels of investigation in theoretical physics. One is primarily descriptive, concentrating as it does on useful phenomenological approaches toward the most economical classifications of large classes of experimental data on particular phenomena. The other, whose thrust is explanatory, has as its aim the formulation of those underlying hypotheses and their mathematical representations that are capable of furnishing, via deductive analysis, predictions - constituting the particulars of universals (the asserted laws)- about the phenomena under consideration.
Concepts
Vector-Tensor Analysis in Relativity Theory
Spinor-Quaternion Analysis in Relativity Theory
The Matter Field Equations
The Electromagnetic Field Equations
The Gravitational Field Equations and Unification with Inertia and Flectromagnetism
Astrophysics and Cosmology