  Paczynski realized that a properly chosen gravitational potential may
accurately model (in a "pseudo Newtonian" theory) general relativistic effects
that determine motion of matter near a non-rotating black hole. Paczynski's
choice, known today as the "Paczynski-Wiita potential", proved to be very
practical. It was used by numerous researchers in the black hole accretion
theory, and became a standard tool in relativistic astrophysics. The model is
an example of Paczynski's admired ability to invent "out of nowhere" simple
ideas that were brilliant, deep and useful. Paczynski has guessed intuitively
the form of the potential. However, it could be also derived by a a
step-by-step formal procedure. I show the derivation here. My derivation is
based on a standard definition of the relativistic "effective potential" in the
Schwarzschild spacetime. The relativistic effective potential may be uniquely
divided into its "gravitational" and "centrifugal" part. The gravitational part
differs from the Paczynski-Wiita potential only by a constant.
