a set is a collection of things (roughly speaking). the empty set is the collection of things that happens to contain no things. it is represented by the symbol ∅, which i use as a symbol for the penumbra.
for any logical claim p, it is trivially true that 'p is true for every member of the empty set'. this can be written ∀x∈∅,p(x). for members of the empty set (of which there are none), everything is true (and false). this is why magic is possible.
the boundary set of a given set is the set of things that are on the 'edge' of the set, for some appropriate measure. the boundary of the empty set is also the empty set, regardless of chosen way to measure that, but it's a different way of looking at it. the set of miracles is the empty set. the set of things that are miracles and also not is the boundary set of the empty set, where it interfaces with everthing else. for the purposes of the penumbra metaphor, the boundary set of the empty set is empty in a different way to the empty set itself, by being an infinitely thin margin rather than merely a universal negation.