The heart wants a constant care to keep it from getting rusted, for the nature of this life of illusion is such that some little unimportant things, which are not of the least value, coming from the outer life, the heart may be affected by, and the rust may be produced as the mere touch of water can produce rust upon the iron. Once the feeling has become soured it is as difficult, if not impossible, to turn it sweet again as to make sour milk sweet.