Tsagalis, Christos. 2012. From Listeners to Viewers: Space in the Iliad. Hellenic Studies Series 53. Washington, DC: Center for Hellenic Studies. http://nrs.harvard.edu/urn-3:hul.ebook:CHS_TsagalisC.From_Listeners_to_Viewers.2012.
Part III. Paratopic Space: Similes and Visual Imagery
The important observation here is that the παραβολή is a form of comparison made to serve another purpose and not required on its own account, a point made explicitly in Aristotle’s Topics (156b25–27).  Although the illustrative parallel is employed to persuade or prove, it is fair to say that its core aspect is analogy based on knowledge, even knowledge of potential and recognizable situations, devoid of any historical element.
Although this description of εἰκασία and παραβολή is included in the discussion of “grand” style (μεγαλοπρεπὴς χαρακτήρ), it is also applicable to “elegant” style (γλαφυρὸς χαρακτήρ), while παραβολή also pertains to “plain” style (ἰσχνὸς χαρακτήρ) and εἰκασία to “forceful” style (δεινὸς χαρακτήρ).  The main point made by the author of On Style, despite reasonable objections concerning his omitting any discussion of εἰκασία from the plain style and treating παραβολή as unsuitable for the forceful style, is that he has brought into the picture the features of length and poetic suitability of the παραβολή.  In fact, some of his arguments, like the one that “the comparison owes its vividness to the fact that all the accompanying circumstances are mentioned and nothing is omitted” (209 τὸ γὰρ ἐναργὲς ἔχει ἐκ τοῦ πάντα εἰρῆσθαι τὰ συμβαίνοντα, καὶ μὴ παραλελεῖφθαι μηδέν),  draw on specific Homeric examples of extended similes,  which silently shows that the author set out to discuss not the single feature of simile but two distinct types of literary comparison, εἰκασία and παραβολή. 
The very language of this definition shows that the simile aims at helping listeners or readers to “visualize” mental images by means of what is familiar, and therefore easily retrieved by the mind’s eye, as well as what is pictureable. While testifying to one of the principal functions of the simile, the expressions εἰς ὄψιν ἄγειν ‘lead into one’s sight’  and δι’ ὁμοίων καὶ γινωσκομένων ‘by means of similar and known things’ accentuate the importance of visual representation through what is identifiable and common to human experience. To return briefly to the term paratopic space, I suggest that the delineation of simile space in parallel with but also beyond that of ordinary story space is based on these two elements of the definition of the simile, that is, “visualization of mental images” and “familiarity or knowledge.” As we will now see, it is the kind of “knowledge” employed in such mental representations that plays a crucial role in creating the paratopic space of the simile. In this light, let us see how ancient rhetorical theory distinguished between two kinds of such “literary” knowledge:
One of Trypho’s most important observations concerns the presentation of the subject within the simile as “performing some activity.” The very formulation of the term “activity” (ἐνέργειά ἐστι φράσις ὑπ’ ὄψιν ἄγουσα τὸ νοούμενον, οἷον μυρίοι, οὐκ ἄνδρεσσιν ἐοικότες, ἀλλὰ Γίγασιν· ἔχονται δὲ τῆς ἐνεργείας καὶ αἱ τοῦ Ὁμήρου παραβολαί)  suggests that Trypho is here describing a type of illustrative comparison based on the subject’s performing some action, and that this kind of situational comparison based on extended analogy “brings thoughts before the mind’s eye.”  Trypho therefore suggests that Homeric similes (αἱ τοῦ Ὁμήρου παραβολαί), which are marked by this type of illustrative comparison, enhance the “imaging” of mental pictures by presenting the audience with vivid visualizations of νοούμενα. But what else can these νοούμενα be than the narrative scenes or events that the storyteller decides to present by means of extended similes?