It is difficult to get a man to understand something, when his salary depends upon his not understanding it
-- Upton Sinclair

Why "... his not understanding it", and not "... him not understanding it"?

I thought that the meaning was "... when his salary depends about (the man) not understanding (something)"


Your comprehension is correct. Here "his" is used because "the man not understanding something" is considered as an "attribute" of the man.

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