 Some constraints are flexible, and these make possible an important feature of living systems—the ability of organisms to control production mechanisms (such as those involve in fermentation or muscle contraction) through the actions of  control mechanisms. On Pattee’s account, control mechanisms change the flexible constraints in production mechanisms in light of information that is procured by making measurements. Negative feedback control mechanisms, such as thermostats, are simple examples: the thermostat makes a measurement of a variable (temperature in the room) that is affected by the operation of the production mechanisms—the furnace—and based on the measurement executes action on the constraints in the furnace mechanism. Examples of feedback control mechanisms are widespread in cells. Some of the best-known examples are the lac operon (Jacob &amp; Monod 1961) and feedback control of glycolysis by ATP (Ghosh &amp; Chance 1964). The measurements used by control systems need not be restricted to states affected by the activity of the production mechanism; they can also measure states in the organism or states in its environment. Using such measurements, production mechanisms can be controlled so as to operate only in particular circumstances, enabling the organism, for example, to navigate to a food source or avoid a predator as needed.