In his original 1905 paper on what we call "The Special Theory of Relativity," (SR) Einstein gave as his two postulates: 1) the laws of physics take the same form in every inertial reference system (the Principle of Relativity); 2) the speed of light in free space has the same value, c, in all inertial reference frames (the constancy of light). See A. Einstein, "Zur Elektrodynamik Bewegter Korper," Annalen der Physic 17 (1905): 891-921..There are actually a variety of ways to obtain these postulates from other areas in physics, such as Maxwell’s equations or electromagnetism; cf. J. R. Lucas and P. E. Hodgson, Spacetime & Electromagnetism (Oxford: Clarendon Press, 1990).There is also a vast literature discussing the scientific and philosophical implications of SR. For a helpful guide, see Arthur I. Miller, Albert Einstein's Special Theory of Relativity: Emergence (1905) and Early Interpretation (1905-1911) (Reading, Massachusetts: Addison-Wesley Publishing Company, Inc., 1981).