We should therefore apply this principle to sex. Sex activity is more pure or impure according to the level on which it chiefly functions. A man who traffics with prostitutes is not immoral because he is functioning physically, but because he is functioning almost entirely on the physical level. Purity is really sex emphasis on the highest levels. Ideally the physically sex-act should therefore be a mere expression of spiritual, mental, emotional love; it should just happen. The moment one begins to lay the emphasis on the lower levels, the more one becomes correspondingly less pure. Lust, in fact, is the desire only for physical sex experience--the wrong proportion and balance. A man's stage of moral development is to be discovered by the particular level on which he is most active. We must not avoid the consequence of this principle. We must be prepared to admit as relatively moral, behaviour which popular philosophy might regard as immoral. We must also be prepared to regard as immoral many marriages in which the physical is the chief incentive. The lowest stage of impurity would seem to be reached in cases, whether between man and harlot, or husband and wife, where the physical function is so emphasized that artificial physical aids are invoked in order to excite the physical passions and make them the cause instead of the result of sex emotion and thought. Chapter 4: Celibacy If once the third principle named in the preceding chapter is fully appreciated, we have already laid the foundation of our moral standard. We shall have seen that physical expression is the sacramental form of the invisible and super-physical motive, and that immorality is the shifting of emphasis from higher to lower levels. We shall be in possession of a test by which we may in almost all sex problems determine a comparative virtue or evil of any practice or conduct. Now this involves the equally fundamental theory that sex is not entirely a physical activity. In the popular conception sex is always confused with physical sex expression. But this conception, I submit, is entirely inaccurate. Even though Freud may be justifiably criticised for straining the word "sex" to include many forces which do not directly tend to incite physical sex activity, he has successfully shown that sex is the motive behind emotions and conduct which would not popularly be termed sexual at all. A musician may, for example, be drawing on his sex-energy in composing or performing musical works; a humanitarian may be sexual in his diffused love of fellow-men and women. We cannot possibly draw a line and say that here sex begins and there it ends. We can only admit that it carries far beyond those particular physical manifestations which we popularly associate with sex.