Between a sign and the thing it signifies there is the fixed, determined relationship of cause and effect. We see this in the case of the footprint in the sand, the tear on the eyelash, or the trademark of a commercial product. But no matter how closely tied a symbol is to the thing symbolized, the relation is variable, flexible, and free. It is in poetry, however, that the symbolic value of words reaches its apex. The cross has become the symbol of Christianity not because of its form but because the Christians, following St. Paul, at a definite moment in their history, decided to adopt the instrument of Christ's torture as their emblem. Similarly, the relation between a word and its meaning depends on its origin, its history, and its usage.