This chapter analyses the concept of function and how to avoid teleological standpoints and the machine analogy. Functions are not natural facts: they are not natural laws, but they are quasi-teleological since they are relative to our sets of values. They are hybrid-concepts since they are empirical and normative. One distinguishes functional role from functional capacity. Dysfunctions define functions counterfactually. The causal structure of physiological processes is intrinsic to the organism, but functions are never intrinsic but always relative to an observer. Physiologists describe functions just like physicists describe the natural gas laws, so that physiology is false. Malfunctions define functions counterfactually. Diseases are usually not malfunctions.