  Tableaux originate as a decision method for a logical language. They can also
be extended to obtain a structure that spells out all the information in a set
of sentences in terms of truth value assignments to atomic formulas that appear
in them. This approach is pursued here. Over such a structure, compositional
rules are provided for obtaining the presuppositions of a logical statement
from its atomic subformulas and their presuppositions. The rules are based on
classical logic semantics and they are shown to model the behaviour of
presuppositions observed in natural language sentences built with {\em if
\ldots then}, {\em and} and {\em or}. The advantages of this method over
existing frameworks for presuppositions are discussed.
