Truth is a virtue that stands aside when life is at stake. That is why Avraham could lie to save his life when asked whether Sarah was his wife (Gen. 12:12, 20:2). But a detail of his justification raises larger questions about situations when truth is not an option.
On confrontation by Avimelekh over his lie that Sarah was his sister, Avraham explained his action with two reasons:
And Avraham said, “(1) Because I thought, surely the fear of God is not in this place; and they will kill me on account of my wife. (2) But indeed she is truly my sister. She is the daughter of my father, but not the daughter of my mother; and she became my wife.” (Gen. 20:11-12)
Avraham was scared that Avimelekh would kill him and, additionally, as a cousin Sarah can be legitimately called his sister. Why did Avraham offer the second explanation? Fear of death, piku’ach nefesh, is a sufficient justification for the lie. The second reason seems unnecessary.
I’d like to suggest an answer within the realm of anachronistic halakhic midrash. Halakhic authorities debate the nature of the prohibition to avoid lies. The Torah commands “mi-dvar sheker tirchak, distance yourself from lies” (Ex. 23:7). The language is unusual. It does not say “Do not lie” but instead that you must distance yourself from lying. Does it mean that lying is not only forbidden but so improper that you must take extra precautions to stay as far away from it as possible? Or is it less than an absolute prohibition, only requiring distance and not complete abstention?
The Rambam (Mishneh Torah, Hilkhos Dei’os 14:13) writes that a Torah scholar “does not alter the truth in his speech, not adding or subtracting, except for matters of peace and the like.” Later scholars infer that in matters of peace, you should preferably add or subtract rather than lie. In other words, you must stray only a minimum from the truth. However, Tosafos (Kesubos 17a sv. kallah & yeshabchenu) say that the permission to praise an ugly bride (for the sake of peace) extends even to falsely saying that she is beautiful, rather than requiring finding an honest praise.
Commentators explain that according to Tosafos, in situations where lying is permitted the prohibition is entirely non-existent. You do not have to minimize the non-truth. However, according to the Rambam, in such a situation the prohibition exists but is overridden. Therefore, according to the Rambam you must minimize the lie while according to Tosafos that is unnecessary (see R. Daniel Z. Feldman, The Right and The Good, pp. 79-81).
According to Tosafos, Avraham’s second explanation is puzzling because once piku’ach nefesh permitted lying, it is entirely allowed. He had no need to speak about Sarah with technical accuracy because the situation allowed for lying. However, according to the Rambam, the prohibition was overridden but still applicable and therefore Avraham still needed to minimize his lying. His reference to Sarah as his sister/cousin was accurate.
Indeed, Avraham’s second answer may be seen as teaching this very rule: Truth is so important that even when running for your life you need to remain as close to truth as possible.