  Arveson's extension theorem asserts that B(H) is an injective object in the
category of operator systems. Calling every self adjoint unital subspace of a
unital *-algebra, a quasi operator system, we show that Arveson's theorem
remains valid in the much larger category of quasi operator systems. This shows
that Arveson's theorem as a non commutative extension of Hahn-Banach theorem,
is of purely algebraic nature.
