This paper gives a direct
constructive proof of the spectral theorem for a normal operator T (bounded or
unbounded) in a complex Hilbert space. It depends on the results, recently obtained
by elementary methods, that an unbounded positive self adjoint operator A has a
unique positive self adjoint square root A1∕2; and an arbitrary self adjoint
operator A has a unique representation A = A+− A− with A+ and A− self
adjoint and positive and the range of each contained in the null space of the
other.