We recall that Aristotle mentions the arousal of pity and fear as the main purpose of tragedy and claims that the tragic poet must aim to arouse such emotions in the audience primarily by means of the plot. In discussing thought, he mentions that agents may arouse emotions in one another by means of language. We find, then, an interesting parallel between the tragic poet and the characters he creates. The plot is an implicit means of arousing emotion employed by the poet, and thought is an explicit means of arousing emotion employed by the agents of the plot.
In discussing metaphor, Aristotle's classifications are of some interest, though he seems to have a rather limited sense of what a metaphor is and how it works. He speaks of metaphor as if it were an extra spice that can be sprinkled on top of whatever is being expressed on a literal level. He values metaphor because it can raise poetry above the humdrum of everyday speech but worries that too much use of metaphor can inhibit clarity.
One might first remark that metaphor is not simply and extra frill added to speech and that it serves the purpose of enhancing clarity, rather than detracting from it. "Juliet is the sun" gives us a much more vivid and clear understanding of Romeo's feelings than if he'd simply said, "Juliet is very beautiful" (See the SparkNote for Romeo and Juliet). This raises a second question, whether metaphor can be considered a simple matter of substituting one word for another. Calling Juliet "the sun" says a great deal—that she is radiant, that she is the source of all life, that she warms Romeo, etc.—and it is far from clear how this simple metaphor could be translated into "literal" speech. Some metaphors are even impossible to translate into literal speech.
Last, we might observe that it is almost impossible to speak without using any metaphors at all. Aristotle himself gives us unwitting evidence of this fact when discussing the metaphorical "Here stands my ship." He says that "stand" is used as a metaphor for "lying at anchor," when, of course, "lying" is itself a metaphor. This is somewhat a matter of translation, but it can often be very difficult to find a non-metaphorical usage. Our emotional vocabulary, for instance, is almost all metaphorical. Words like "upset," "confused," "hurt," "moved," and "touched" all borrow from expressions of physical states, and there is no non-metaphorical equivalent. Metaphorical usage is so intrinsic to our language-using capabilities that it is often very difficult to determine when we are speaking literally and when we are using metaphor.