For man two wings are necessary. One wing is physical power and material civilization; the other is spiritual power and divine civilization. With one wing only, flight is impossible. Two wings are essential. Therefore, no matter how much material civilization advances, it cannot attain to perfection except through the uplift of spiritual civilization.
- 'Abdu'l-Bahá (‘The Promulgation of Universal Peace: Talks Delivered by 'Abdu'l-Bahá during His Visit to the United States and Canada in 1912’; The Compilation of Compilations, Vol. III, Social and Economic Development)