On 13 November 866, Pope Nicholas I wrote to King Boris I of Bulgaria: "Whether you or your women wear or do not wear trousers neither impedes your salvation nor leads to any increase of your virtue"[11] (sive vos, sive feminae vestrae, sive deponatis, sive induatis femoralia, nec saluti officit, nec ad virtutum vestrarum proficit incrementum - Patrologia Latina, CXIX, 1002).[12] Some members of the Society of Saint Pius X have spoken of the preference of women's wearing skirts rather than trousers.[13] Cardinal Siri's letter has also been cited as justification for women wearing skirts and dresses.[14] In addition, Saint Thomas Aquinas, the Church's principal theologian, also taught that "outward apparel should be consistent with the estate of the person, according to the general custom. Hence it is in itself sinful for a woman to wear man's clothes, or vice versa; and it is expressly forbidden in the Law (Deuteronomy 22)..." [15]