  Gentzen designed his natural deduction proof system to ``come as close as
possible to actual reasoning.'' Indeed, natural deduction proofs closely
resemble the static structure of logical reasoning in mathematical arguments.
However, different features of inference are compelling to capture when one
wants to support the process of searching for proofs. PSF (Proof Search
Framework) attempts to capture these features naturally and directly. The
design and metatheory of PSF are presented, and its ability to specify a range
of proof systems for classical, intuitionistic, and linear logic is
illustrated.
