The common concept of sex has consisted of many factors, like culture, tradition, scientific fact, and personal experiences and so on. As a result, it includes several prejudices. To explain the true nature of sex, we may have two different types of cause, ultimate and proximate causes, which the variational model of the modern evolutionary theory has and which we cannot find in the usual normal state model. And why-type question is generally answered by using the ultimate causes, and how-type question by using the proximate causes.
Using two different types of cause, we explain the development, the structure and the adaptation of sex. In particular, the adaptive explanation of sex, consisting of the explanation of the existence of sex, sexual selection, and the sex ratio, shows not only the mystery of sex but also the emergent meaning of sex.
Then we analyze the philosophical meaning of two different types of cause and their relevant problems. Genetic determinism and naturalistic fallacy are interpreted from the ultimate point of view. Lastly the convention, which is introduced as a local law in biology, is clarified naturalistically.
