Figure of thought
Ancient rhetorical theory distinguished between form and content. As Gordon Williams states in introducing his study on Figures of Thought in Roman Poetry, "Language was subject to ordering by exhaustive description of vocabulary, syntax, and figures. The content was likewise subject to ordering by the rules of inventio (the technique by which all the latent or inherent possibilities in a given idea or cluster of ideas could be 'discovered' and exploited)."
Figures of speech, however, are those we most commonly hear about, and virtually any reader can retrieve a definition of 'figure of speech', as in fact the existence of a Wikipedia entry for 'figure of speech' demonstrates. When it comes to describing a figure of thought, the task is not as comfortable as we might wish, especially when our primary area of inquiry is poetry rather than a prose composition, which adheres to the rules of rhetorical theory that were for the most part created in their fullest articulation in the age of Quintilian. In their approach to language, the Roman poets (Horace, Catullus, Propertius, Tibullus) did not distinguish between inventio and elocutio, which explains somewhat our confusion between figures of thought and figures of speech. The latter are exemplified most immediately by conceptual substitutions, such as metaphor or synecdoche, but if we consider not just the mere linguistic aspect of the substitution and focus most closely on the concepts themselves, then we might begin to perceive the comprehensiveness of the conceptual for the poet, who operates with words to extend and amplify the potential semantics of poetic output.
In other words, if the poet says 'stars' to mean 'eyes', on the linguistic level we witness a conceptual substitution that plays on the gleaming of stars to describe human eyes, and thereby the object described acquires greater detail and more seductive features. The resulting effect of the substitution, however, also includes a fusion of concepts because the gleaming of stars and the gleaming of eyes are added to one another in thought, and the resulting meaning is a new and more potent concept than the one available in the two separate concepts of 'stars' and 'eyes', for after qualifying the eyes of a beloved as stars, the whole universe becomes readable in the beloved's eyes, and the eyes of the beloved are watching us from above on any given night.