The equality of women and men is stated as a foundational principle rather than a social improvement, with 'Abdu'l-Bahá's image of humanity as a bird whose two wings must be equally developed or it cannot fly. The teaching goes further than formal parity in one practical respect: where a family cannot educate all its children, the daughter is to be educated in preference to the son, on the reasoning that she will be the first educator of the next generation. 'Abdu'l-Bahá also predicted that war would become impossible once women had a full share in the affairs of the world. Against this stands the restriction of membership of the Universal House of Justice to men, which is the faith's most-discussed internal problem: it was stated by 'Abdu'l-Bahá, no reason was given beyond an assurance that the wisdom of it would become evident, and the House has stated that it has no authority to alter it. Bahá'ís generally acknowledge the tension openly rather than explaining it away.