In poetry a metaphor differs from a simile in form only not in substance. In a simile the two subjects are kept distinct in the expression, as well as in the thought; in a metaphor, the two subjects are kept distinct in the thought only, not in the expression. An allegory differs from a metaphor; and a figure of speech differs from both. An allegory is one thing figured to be another; it consists in choosing a subject having properties or circumstances resembling those of the principal subject; and the former is described in such a manner as to represent the latter. In a figure of speech, there is no fiction of the imagination employed, as in a metaphor; nor a representative subject introduced, as in an allegory. It regards the expression only, not the thought, and may be defined, the using a word in a sense different from what is proper to it.