The contributions of the great physicist Ludwig Boltzmann to philosophy and biology are not known sufficiently. In philosophy, he was a realist, and much opposed to his colleague's, Mach's, positivism, but also to Berkeley's, Kant's, Hegel's and Schopenhauer's idealisms. In biology, Boltzmann was a passionate Darwinist and tried to explain on the basis of evolution the meaning of photosynthesis as well as the origin of life and of the mind. Boltzmann argued for evolutionary epistemology. Opposing Kant, he derived the fundamental ideas of space, time and causality from the experience of mankind and its ancestors. The laws of thought were acquired in evolution. While they must be broadly true they need not be faultless. Antinomies arise when thought overshoots the mark, i . e. exceeds the limits of the area for which it evolved. As for Boltzmann the experience of the organism in the course of evolution is the only and exclusive source of knowledge, he may be called an absolute Darwinist. A short account of Boltzmann's Darwinian ideas on the origin of morality, on the sense of beauty and on happiness is also given.