Full Idea
Contradictoriness is the boundary both of what is possible and also of what is imaginable.
Gist of Idea
Contradictoriness limits what is possible and what is imaginable
Source
Alan Sidelle (Necessity, Essence and Individuation [1989], Ch.4)
Book Reference
Sidelle,Alan: 'Necessity, Essence and Individuation' [Cornell 1989], p.89
A Reaction
Of course we may see contradictions where there are none, and fail to grasp real hidden contradictions, so the two do not coincide in the practice. I think I would say it is 'a' boundary, not 'the' boundary.