  These doctrines are not, however, the doctrines of [SQUARE]. The doctrines of [SQUARE] are worded entirely in terms of the possibilities of truth values, not in terms of entailment. So “entailment” is irrelevant to [SQUARE]. It turns out that Strawson’s revision of truth conditions does preserve the principles of SQUARE (these can easily be checked by  cases),[27]  but not the additional conversion principles of [SQUARE], and also not the traditional principles of contraposition or obversion. For example, Strawson’s reinterpreted version of conversion holds for the I form because any I form proposition entails its own converse: if ‘Some A is B’ and ‘Some B is A’ both have truth value, then neither has an empty subject term, and so if neither lack truth value and if either is true the other will be true as well. But the original doctrine of conversion says that an I form and its converse always have the same truth value, and that is false on Strawson’s account; if there are As but no Bs, then ‘Some A is B’ is false and ‘Some B is A’ has no truth value at all. Similar results follow for contraposition and obversion.