Allegory as a rhetorical figure

Allegory is one of the classical rhetorical tropes such as synecdoche, metaphor, or irony. Quintilian defined allegory as an extended metaphor (continua metaphora). Allegory presupposes at least two levels of reference: a primary or literal level of meaning which is substituted by an analogous secondary level. For instance, a greenhouse can be understood as a metaphor of a state or nation. As soon as I extend this metaphor by adding further details, such as a gardener and various forms of green, black, and red vegetables, the result can be a political allegory of a parliamentary system. Many allegories are cultural conventions, e.g. the figure of a blind-folded lady holding a pair of scales in her hands is interpreted a personified allegory of justice. The relation between the two levels of understanding can be so obscure or enigmatic that the result is a riddle.