The realm of the unknown is peopled, in the imagination, by monstrous beings and mysterious events with which superstitious fear is readily associated.
Such fear is characteristic of isolated tribal societies, although many such beliefs prevailed in Europe until the 18th century. Fear of this kind is common in children. In adults fear of the unknown is more closely associated with fears of personal security in which the monstrous beings are replaced by muggers and people of unpredictable habits from other cultures. In the elderly such fear tends to be associated with difficulties of dealing with unknown new technologies calling for particular skills.
Given any problem containing 'n' equations, there will always be 'n + 1' unknowns.