  The paper proposes a derivation system for a logic of presuppositions as
introduced by P. F. Strawson. It is based on truth-relevant logic described by
M. Richard Diaz in 1981. In another paper I outlined a derivation system for
t-relevant logic based on truth trees. The conclusion was that a tautology is
truth-relevant iff all the variables in a tree are self-contradicted. It is
possible that a tree terminates without all the variables self-contradicting
themselves. In this case the pertaining formula is still a tautology, but not a
truth-relevant tautology. This concept is extended to the predicate calculus,
i.e. a logic of presuppositions or a variant thereof.
