In simpler terms, what's this quote saying?

"some have felt that these blundering lives are due to the inconvenient indefiniteness with which the supreme power has fashioned the natures of women: if there were one level of feminine incompetence as strict as the ability to count three and no more, the social lot of women might be treated with scientific certitude."

The sentence of the OP suggests that women's level of 'feminine incompetence' is not
simply stated/determined, as we could if you asked, "Can the person count to three."
To such a question one can say, "Yes they have that competence." But in the case
at issue, women's abilities to deal with and overcome social obstacles, there are all kinds
of levels; further this issue is not intrinsic to the woman, but relates to her circumstances.

She is not really talking in our sense of 'competence', but rather whether women
will be able to suceed in the face of social complexities and difficulties.

The passage ends with

Here and there is born a Saint Theresa, foundress of nothing, whose loving heart-beats and sobs after an unattained goodness tremble off and are dispersed among hindrances, instead of centring in some long-recognizable deed.

"dispersed among hindrances" is related to this 'incompetence' which is, in other words
insufficient, luck, resources, and opportunities. Saint Theresa (Teresa) is one of those rare successes.

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In my opinion, Joan's line suggests somewhat of a misunderstanding. (That's my impression at least.) The writer is NOT saying women lack a basic competence comparable to the ability to count three.

Joan in part (post #2):
If women were all equally incompetent (unable to count to three),

Writers who used greek and latin literature to perfect their literary forms belonged to the neoclassical realism classical period of literature. can i get brainliest