Human beings are used to understanding their orientation relative to the earth using their senses: vision, vestibular system and proprioception. When in flight only a pilot's vision is usable, and that not always so. But pilots tend to revert to their vestibular and proprioceptive systems automatically when faced with a degraded visual environment whereas only the aircraft's instruments are a reliable indication of their orientation.
Our training provides pilots with situations in which they can misconstrue their orientation, insidiously and persuasively, to remind them of the importance of using their instruments, not to trust their (very fallible) senses.
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