A footnote added in 1915 to Freud's Three Essays on Sexuality:It is essential to understand clearly that the concepts of 'masculine' and 'feminine', whose meaning seems so unambiguous to ordinary people, are among the most confused that occur in science. It is possible to distinguish at least three uses. 'Masculine' and 'feminine' are used sometimes in the sense of activity and passivity, sometimes in a biological, and sometimes, again, in a sociological sense. The first of these three meanings is the essential one and the most serviceable in psycho-analysis. When, for instance, libido was described... as being 'masculine', the word was being used in this sense, for an instinct is always active when it has a passive aim in view. The second, or biological, meaning of 'masculine' an 'feminine' is the one whose applicability can be determined most easily. Here 'masculine' an 'feminine' are characterized by the presence of spermatozoa or ova respectively and by the functions proceeding from them. Activity and its concomitant phenomena (more powerful muscular development, aggressiveness, greater intensity of libido) are as a rule linked with biological masculinity; but they are not necessarily so, for there are animal species in which these qualities are on the contrary assigned to the female. The third, or sociological, meaning receives its connotation from the observation of actually existing masculine and feminine individuals. Such observation shows that in human beings pure masculinity or femininity is not to be found either in a psychological or a biological sense. Every individual on the contrary displays a mixture of the character-traits belonging to his own and to the opposite sex; and he shows a combination of activity and passivity whether or not these last character-traits tally with his biological ones.