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. (October 15, 2018). http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/vimokkha
"Vimokkha." The Concise Oxford Dictionary of World Religions. . Retrieved October 15, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/vimokkha