Assuming the function ψ to be analytic (i.e. differentiable in some domain of the complex plane), one may expand in a Taylor series about x : ψ ψ ( x − − ε ε ) = ψ ψ ( x ) − − ε ε d ψ ψ d x so for infinitesimal values of ε : T ( ε ε ) = 1 − − ε ε d d x = 1 − − i ℏ ℏ ε ε ( − − i ℏ ℏ d d x )