Since then the process of mathematical synthesis in science has been carried many stages further. The exponents of this aspect of scientific progress, of whom we may take the late M. Henri Poincare as the leading representative in our generation, are perfectly justified in treating this gradual mathematical unification of knowledge with pride and confidence. They have solid achievement on their side. It is through science of this kind that the idea of universal order has gained its sway in man's mind. The occasional attacks on scientific method, the talk one sometimes hears of 'breaking the fetters of Cartesian mechanics', seem to suggest that the great structure which Galileo, Newton, and Descartes founded is comparable to the false Aristotelianism which they destroyed. The suggestion is absurd: its chief excuse is the desire to defend the autonomy of the sciences of life, about which we have a word to say later on. But we must first complete our brief mention of the greatest stages on the mechanical side, of which a full and vivid account may be found in such a book as M. Poincare's _Science et Hypothese_. Early in the nineteenth century a trio of discoverers, a Frenchman, a German, and an Englishman, established the theory of the conservation of energy. To the labours of Sadi Carnot, Mayer, and Joule is due our knowledge of the fact that heat which, as a supposed entity, had disturbed the physics and chemistry of the earlier centuries, was itself another form of mechanical energy and could be measured like the rest. Later in the century another capital step in synthesis was taken by the foundation of astrophysics, which rests on the identity of the physics and chemistry of the heavenly bodies with those of the earth.