A simile, to be perfect, must both illustrate and ennoble the subject; must show it to the understanding in a clearer view, and display it to the fancy with greater dignity; but either of these qualities may be sufficient to recommend it.... That it may be complete, it is required to exhibit, independently of its references, a pleasing image; for a simile is said to be a short episode.

prefaces biographical and crirical to the works of the english poets (ed. 1781)