Consider a tautology of the form formula_20. The tautology is true for every choice of formula_21, and after fixing formula_21 the evaluation of formula_23 and formula_24 are independent because they are defined on disjoint sets of variables. This means that it is possible to define an "interpolant" circuit formula_25, such that both formula_26 and formula_27 hold. The interpolant circuit decides either if formula_28 is false or if formula_29 is true, by only considering formula_21. The nature of the interpolant circuit can be arbitrary. Nevertheless, it is possible to use a proof of the initial tautology formula_20 as a hint on how to construct formula_32. A proof systems "P" is said to have "feasible interpolation" if the interpolant formula_25 is efficiently computable from any proof of the tautology formula_34 in "P". The efficiency is measured with respect to the length of the proof: it is easier to compute interpolants for longer proofs, so this property seems to be anti-monotone in the strength of the proof system.