Second: the Three Doors to Deliverance (vimokkha-mukha). When a person resolves to attain nirvāna, he cannot make nirvāna itself a subject of meditation, for it is ‘inconceivable’, not an object of thought. He therefore makes his way towards nirvāna by making impermanence (anicca), suffering (dukkha), and noself (anatman), respectively, his subjects of meditation.
"Vimokkha." The Concise Oxford Dictionary of World Religions. . Encyclopedia.com. (August 18, 2018). http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/vimokkha
"Vimokkha." The Concise Oxford Dictionary of World Religions. . Retrieved August 18, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/vimokkha