A weakly closed algebra of
operators on a Hilbert space is reductive if every subspace which is invariant for the
algebra reduces. If 𝒜 is a reductive algebra, let ℐ be the von Neumann algebra
genenerated by the projections which commute with 𝒜. If ℐ is properly infinite, or it
ℐ has a cyclic vector, then 𝒜 is self-adjoint. If ℐ has no direct summand which is
abelian and of infinite uniform multiplicity, then ℐ is the commutant of
𝒜.