Faculty X is one of the concepts that we can talk to Rebecca about when we first make her acquaintance in the Mansion's Study, after successfully manoeuvring through the inhospitable grounds of the Mausoleum.
Rebecca: Faculty "X" is the mental power used to loosen the hold which our present surroundings have on our attention. The power can also be used to become aware of other places, other times, and other realities.
Apparently, the term was coined and formulated by English writer and philosopher Colin Wilson, which he saw as leading to an increased sense of meaning, and on abilities such as telepathy and the awareness of other energies. In his book The Occult: A History, published in 1971, he suggests that
Faculty X is simply that latent power that human beings possess to reach beyond the present. After all, we know perfectly well that the past is as real as the present, and that New York and Singapore and Lhasa and Stepney Green are all as real as the place I happen to be in at the moment. Yet my senses do not agree. They assure me that this place, here and now, is far more real than any other place or any other time. Only in certain moments of great inner intensity do I know this to be a lie. Faculty X is a sense of reality, the reality of other places and other times, and it is the possession of it — fragmentary and uncertain though it is — that distinguishes man from all other animals.