Immortality has been the core practice objective of Taoist Neidan (Taoist yoga or meditation). If human can become immortals, why we don't see Immortals walking around? Are these then false claims? If yes, then why generations of Taoists in China keep practicing Neidan with the objective to become an Immortal someday? Are they simply stupid? I shall try to answer these questions here.
Zen Master Liu Huang Yang 柳華陽(author of Hui Ming Jing)'s student Zen Master Liao Kong 了空 talked about this issue at the prefer of Taoist Yoga 性命法诀明指, an important classic by Master Zhao Bichen 趙避塵, translated into English by Zen master Charles Luk (unfortunately, the preface pieces have not been included in the English translation). According to Liao Kong, there are two kinds of Immortal: Earthly Immortals (地仙) who live forever on earth and Heavenly Immortals (天仙) who live forever in heaven.
The first stages of Taoist yoga is to train one into Earthly Immortals and there are stages of progress here. At the final stages overlapping with the more advanced Heavenly Immortal practice, one can indeed, according to Master Liao, become Earthly Immortal. But the trick is, by that time a practitioner would have passed many stages of the the more advanced Heavenly Immortal practice. And since such Earthly Immortals are usually enlightened sage, they will surely aim at a higher objective, i.e. to continue their practice and become finally a Heavenly Immortal. What is a Heavenly Immortal? He is someone who will discard his earthly body to join the family of heavenly immortals. In order words, he will choose to die!
You may wonder: how about those practitioners who willingly choose to become Earthly Immortals instead of Heavenly Immortals? Simple. They (will) die before they fail to achieve their objective (of course, they can't complain; and their students won't complain either because through Taoist yoga they and their masters live longer and healthier lives).
OK, there is no physical Earthly Immortal that we can see, it doesn't matter, because Taoism is a religion, the most important objective will be to become Heavenly Immortals anyway, so how can such concept be interpreted to our better understanding?
To answer this question, we shall make use of the concept of psychic reality and metaphysical reality as used by Carl Jung. Why do Taoists believe in the existence of and the possibility of becoming Heavenly Immortals? The answer is simple. Taoist yoga practitioners truly encounter previous Immortals (most noticeably Grandmaster Lu Zu (呂祖) during meditation or dream. These are their psychic reality, can be studied/analyzed, and cannot be refuted (unless we say they are all lying which I think is not possible). Can we physically prove the existence of heavenly Immortals? No, and the question is as futile as try to physically prove the existence of God. As Jung said if metaphysical beings can be detected by our scientific measuring instruments they will be the subject matter of physical science rather than metaphysics.
Needless to say further speculations into the metaphysical area belong to the arena of the faithfuls. As a concluding remark, since we can't perceive Immortals with our physical senses, assuming there does exist such Immortals, the only way we can communicate with them, or they communicate with us, is through our psyche, i.e. during deep meditation or in dreams. And I rest my case here.