 Also, he argues, this primary heuristic is sometimes at odds with a secondary heuristic—acquiring conditional beliefs by testimony. Often this is unproblematic, but, as Gibbard (1981) showed, two people with different background knowledge can flawlessly come to opposite judgements about whether if \(A, B\). They then convey their judgements to a third person, who trusts them, but who cannot accept both judgements, using the suppositional procedure. 