Second Sight

Second sight is a mystical sense, or rather set of senses – though the one analogous to vision is the most common one. About 95% of the fae have at least basic second sight, and some have developed it further (to some extent it can be developed by practice, though there may be genetic considerations as well).

In the world view of the fae, the physical world is a manifestation of a deeper level of reality, and it is this deeper level that second sight perceives. In most cases the two levels are similar, and such slight differences as there are are likely to be overlooked. But some phenomena stand out clearly to second sight. Among these are illusions and transformations, where second sight reveals the object as it should be. "Correspondence points", which are interdimensional weak spots allowing travel between worlds, appear to second sight as a manifestation of "the same place" in two world. Spirits and other invisible creatures can also be perceived, since they have an "underlying" existence if no electromagnetic manifestation. To second sight cold iron looks completely black. (This list of uses may not be exhaustive; more obscure uses may require GM rulings.)

Although usually just called "second sight" or "the Sight" a lot of the sighted have developed similar mystical analogues to hearing, smell, touch, etc.

Being able to perceive the world this way is vital to fae magic. Whether manipulating weather or simply crafting an object of superior quality, one has to be able to see what one is doing in a way most humans can't.

A typical Hero System writeup of Second Sight can be found here.