"The cause of ascent is lightness, and the cause of lightness is heat."

"O friends of God! Incline your inner ears to the voice of the peerless and self-subsisting Lord, that He may deliver you from the bonds of entanglement and the depths of darkness and enable you to attain the eternal light. Ascent and descent, stillness and motion, have come into being through the will of the Lord of all that hath been and shall be. The cause of ascent is lightness, and the cause of lightness is heat. Thus hath it been decreed by God. The cause of stillness is weight and density, which in turn are caused by coldness. Thus hath it been decreed by God."

-Bahá'u'lláh, The Tabernacle of Unity, p. 71