Enthymemes are formed from probabilities and signs that may be sound or unsound. For example, an enthymeme is basically a syllogism with an unstated premise. If we say that Cows are mammals so they suckle their young, we are stating an enthymeme because we take it for granted that mammals suckle their young. However, it is in this aspect of Aristotle's rhetoric that we can take a critical approach. For it is in this unstated premise that we may find room for error or lack of validity in his systematic treatment of rhetoric. For it is extremely difficult to determine that a hidden premise really exists or is "there." For example, any nonsensical argument could be made into a valid one by arbitrary additions. Aristotle points this weakness out when he discusses the difference between fallible and infallible signs "The other kind of Sign, that which bears to the proposition it supports the relation of universal to particular, might be illustrated by saying, ‘The fact that he breathes fast is a sign that he has a fever.' This argument also is refutable, even if the statement about the breathing be true, since a man may breathe hard without having a fever" (Bizzell 156).
Rhetoric falls into three divisions (political, forensic, and the ceremonial oratory of display), all of them determined by the audience of hearers. The listeners makeup the most significant aspect of speech-making because those who hear the speech impact the speech's end and object. Therefore, despite the type of oratory being used, each is designed to bring about an end "Political speaking urges us either to do or not to do something…Forensic speaking either attacks or defends somebody…The ceremonial oratory of display either praises or censures somebody" (Bizzell 157).