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Even so, she might forgive him -- forgive him the sloppiness and the clusimess and the endless laughing and the general annoyance that he is, forgive him for being a child -- had he possessed the one thing for which she has a weakness: brilliance.

Hi,

The above is from the novel "Loser." She refers to Mrs. Biswell, Zinkoff's second-grade teacher, and him refers to Zinkoff.

For a start, I guess to simply to present subjunctive "should he possess" is better than the past subjunctive tense "had he possessed" in the above. Correct me if I am wrong.

Secondly, could you come up with a reason why she has a weakness for owning brilliance? I can't figure it out though I have raked my brain hard. Thanks.
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"had he possessed" seems fine to me. "should he possess" seems not quite right.

She doesn't have a weakness for "owning brilliance". She has a weakness for brilliance (in other words, she evidently very much admires brilliant children). Zinkoff does not possess this quality.
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Angliholicwhy she has a weakness for owning brilliance
"possessed" here is used in the sense of "exhibiting an attribute" as opposed to "owning a thing."

If you check your dictionary it will probably tell you that when used in this sense "possessed" is followed by "of" but in this case "of" is neither needed nor allowed.