I was saying something about typologies; "ornamental" typologies and how they work, though the explanation I gave was a bit sketchy.
Luann Brizenden's The Female Brain is the most visible recent example of this genre, if not the most pernicious. This book makes extravagant false assertions about human psychology, claiming quite illusory differences in the mental pathology of men and women, to the extent of imagining the two sexes as distinct types or species.
The types outlined in The Female Brain have no basis in fact: they're more akin to the types outlined in astrology, but they borrow, as it were, their credibility from the natural sciences.
After Deleuze we can say that there aren't "essential" and "inessential" ideas. There are, however, different ways of conjugating ideas, and so different ways of conceptualising the fact that there are two sexes. These differing conceptions affect the apparent essentiality of there being two sexes. One idea of the sexes has the sexes limited to two sexes: essentially two sexes, as operative in the process of the reproduction of life. But conversely it's possible to conceive the sexes as categories containing no internal factor limiting their proliferation, thereby allowing us to conceive a third sex, a fourth sex, and so on. The two sexes can either be strictly a binary pair or the only extant survivors of a larger set, of which the other members are absent. We don't require differences in fact for this discrepancy to operate, but only in degree of abstraction, assuming essential always some abstraction.
The Female Brain insists its types or species: the supposed "male" and "female" brain, function as a binary pair in the same way as do the sexes, considered as part of the process of reproduction. But these types aren't, in fact, consistant with sexual difference: rather the typology borrows the attribution of an essential binarity from the concept of the two sexes. So, at first glance, the "male" and "female" brain appear as a necessarily binary pair, rather than two arbitrary designations from a limitless set of arbitrary designations. (In this respect Brizenden is more convincing than Jung).
But this also alerts us that The Female Brain is an aesthetic work and not a scientific work. It's really inviting the reader to identify with one type of brain against the other. The designations "male" and "female" brain aren't really credible in relation to an outside world; in terms of evolution, for instance, but only in relation to each other. They are characters in a kind of novel. They're described in a static way and motion is given to the whole thing by the reader questioning the validity of the structure, alternately seduced and repulsed by this description.
This is a caste system. From identifying (himself or presumably) herself with one of the types described by The Female Brain, provisionally and tentatively, the reader is prompted into (what used to be called) "profound meditations" on the possible "divine government of the world." The Female Brain reproduces the affectivity of religious explanations of society, where social institutions are imagined as inevitable, onerous, arbitrary: a strange mixture of necessity a contingency such as is found in de Quincey's ideas about the Hindu system:
"even Englishmen, though not bred in any knowledge of such institutions, cannot but shudder at the mystic sublimity of castes that have flowed apart, and refused to mix, through such immemorial tracts of time"
(this pleasure in anachronism is inherent in all religion)
The Female Brain is an introduction to archaic thinking, though it apparently belongs to a different genre. In some ways it's a counterpart to the recent practice of remodelling the entrances to old buildings in the style of airport architecture (e.g. the National Gallery, London).
It's worth considering Jung's typology alongside Brizenden's because while their respective typologies work in the same way, the "types" they describe are different. The system works independantly of its content.