"And because Being turns out to be analogously predicated of the ten genera, it is divided into diverse modes." (In I Sent. 22, 1, 3 ad 2)
(...) "some things are found to be added to Being, because Being finally boils down to ten [upper] genera, of which each one adds something to Being, not however some accident or difference [is added] residing outside the essence of Being, but a determined mode of Being, which has its foundation in the very existence of the thing." (De Ver. 21, 1)
So while substance can have accidents (added), Being (as such) cannot have accidents added to it from without as it were, because there is nothing outside Being.
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