Biology is in a unique position among the natural sciences. It is not simply complex physics and chemistry, for living organisms have a psychological as well as a physical side. Even as physical systems their character is highly special, largely because their material substance is continually changing; perhaps it was from them that Heraclitus derived his idea that all is flow. The comparison with vortexes and candle flames is an old one. Wilhelm Ostwald included living organisms in his class of “stationary systems”: they represent not a static but a kinetic equilibrium, what we now often call a “steady state”; there is a balance of constitutive and dissipative processes. Materials and energy converge to the living center, undergo there characteristic transformations, among which complex chemical and structural syntheses are conspicuous, and are again distributed at random to the surroundings. The living organism is thus the seat of a special type of physical and chemical activity found nowhere else in nature, and it is significant that this is associated with psychical activity—demonstrably in the higher organisms, by implication in the lower.