  Using Wilson-Polchinski renormalization group equations, we give a simple new
proof of decoupling in a $\phi^4$-type scalar field theory involving two real
scalar fields (one is heavy with mass $M$ and the other light). Then, to all
orders in perturbation theory, it is shown that effects of virtual heavy
particles up to the order $1/M^{2N_0}$ can be systematically incorporated into
light-particle theory via effective local vertices of canonical dimension at
most $4+2N_0$. The couplings for vertices of dimension $4+2N$ are of order
$1/M^{2N}$ and are systematically calculable. All this is achieved through
intuitive dimensional arguments without resorting to complicated graphical
arguments or convergence theorems.
