The first principle referred to in this letter is “the principle that the writings of ‘Abdu’l-Bahá and the Guardian are thoroughly imbued with the spirit of the Revelation of Bahá’u’lláh and intimately linked with the Teachings of Bahá’u’lláh Himself.” See Messages from the Universal House of Justice, 1986–2001, the Fourth Epoch of the Formative Age (Wilmette: Bahá’í Publishing, 2009), pars. 201.9–201.11.
“Social Action: A paper prepared by the Office of Social and Economic Development at the Bahá’í World Centre, 26 November 2012,” published in Framework for Action: Selected Messages of the Universal House of Justice and Supplementary Material, 2006–2016 (West Palm Beach: Palabra Publications, 2017), pp. 327–350. It is also available on the Bahá’í Reference Library.