Having discussed plot and character, Aristotle turns his attention toward thought and then diction (he never specifically addresses melody or spectacle). Aristotle defines thought as everything that is effected by means of language. Thus, when agents try to prove or disprove a point, to arouse emotion, or to inflate or deflate a matter, they are exhibiting thought. Thought is closely linked to rhetoric, and Aristotle points to the more thorough discussion to be found in his writings on that latter subject.
Aristotle divides the subject of diction into eight parts: letter, syllable, conjunction, article, noun, verb, case, and speech. Though many of these terms are identical to our modern uses of them, we should note that Aristotle is concerned less with written language and more with spoken language. As a result, Aristotle treats the letter—the fundamental building block of language—as a unit of sound rather than as a single written character. The concept of case, unfamiliar to English speakers, deals with the different uses of a word. For instance, "with the dog" and "for the dogs" are different cases of "dog," and "walked?" and "walk!" are different cases of "walk." Speech is more like what we would call a clause than a sentence. It does not have to contain a verb, but it must be made up of significant parts.
Chapter 21 is concerned with the structure and uses of the noun, though it is concerned primarily with the uses of metaphor. Aristotle distinguishes four ways metaphor can be used. (1) The genus to species relationship, where a more general term is used instead of a specific term. Aristotle uses the example of "Here stands my ship," where "stand" is a more general way of saying "is anchored." (2) The species to genus relationship, where a more specific term is used in place of a general term. Aristotle's example is "Truly ten thousand good deeds has Ulysses wrought," where "ten thousand" is a specific term representing the more general "a large number." (3) The species to species relationship, where one specific term replaces another. (4) Metaphor from analogy, which consists of substitutions between "x is to y"-type relationships. For instance, old age is to life as evening is to day, so we can speak metaphorically about the "old age of the day" or the "evening of life."
Aristotle concludes his discussion of diction with a few remarks on style. A poet should aim for a middle ground, expressing himself with clarity but without meanness. Aristotle suggests that the use of ordinary words and ordinary language is mean and prosaic. Poetry can be spiced up by the use of foreign or strange terms, metaphor, or compounded words. However, an overenthusiastic use of such devices will render poetry unintelligible. Too many foreign words will make the poetry barbaric and too much metaphor will turn it into a big riddle. The key is to apply these devices in moderation. Of these different devices, Aristotle most values the metaphor, as it cannot be taught but only grasped intuitively. There is a certain level of genius in being able to identify similarities between dissimilar things.
Chapters 19–22 are almost certainly the least interesting part of the Poetics. Thought and diction are far less important to tragedy than are plot and character, and a good deal of the discussion is difficult to follow without an understanding of Ancient Greek. Chapters 20 and 21 in particular, which deal with grammatical questions, seem out of place in the larger context of the work, and many scholars suspect that they are not by Aristotle at all.
As we recall, Aristotle makes a distinction between the character and the thought of an agent. The thought of an agent is everything he or she expresses verbally. This includes persuading, reasoning, and arousing emotion, among other things. We might understand it as the impression an agent consciously tries to make on others. What we might infer from his or her unspoken behavior is more a matter of character.
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.