Doctrine of mental reservation

The doctrine of mental reservation, or of mental equivocation, was a special branch of casuistry (case-based reasoning) developed in the late Middle Ages and the Renaissance, and most often associated with the Jesuits.

Mental reservation is a form of deception which is not an outright lie. It was argued for in moral theology, and now in ethics, as a way to fulfill obligations both to tell the truth and to keep secrets from those not entitled to know them (for example, because of the seal of the confessional or other clauses of confidentiality). Mental reservation, however, is regarded as unjustifiable without grave reason for withholding the truth. This condition was necessary to preserve a general idea of truth in social relations.

In wide mental reservation, equivocations and amphibologies are used to imply an untruth that is not actually stated. In strict mental reservation, the speaker mentally adds some qualification to the words which they utter, and the words together with the mental qualification make a true assertion in accordance with fact.

A frequently cited example of equivocation is a well-known incident from the life of Athanasius of Alexandria. When Julian the Apostate was seeking Athanasius's death, Athanasius fled Alexandria and was pursued up the Nile. Seeing the imperial officers were gaining on him, Athanasius took advantage of a bend in the river that hid his boat from its pursuers and ordered his boat turned around. When the two boats crossed paths, the Roman officers shouted out, asking if anyone had seen Athanasius. As instructed by Athanasius, his followers shouted back, "Yes, he is not very far off." The pursuing boat hastily continued up the river, while Athanasius returned to Alexandria, where he remained in hiding until the end of the persecution.

...
Wikipedia