Husband and wife are two souls in the one flesh...

Abstract

[Extract] It hath beene resolved by the Justices that a wife cannot be produced either against or for her husband...[for they are two souls in one flesh], and it might be a cause of implacable discord and dissention betweene the husband and the wife, and a meane of great inconvenience. (Lord Coke, 1628, cited in David Lusty, 2004)

As a result of the Biblical principle that husband and wife are 'two souls in one flesh', there is a common law tradition whereby women may not be obliged to give evidence against their husbands. This principle has apparently extended to apply to men giving evidence against their wives.