Consider the argument:
15) I amhungry; therefore I amhungry.
Intuitively this should count as valid. But suppose we thought of the components of arguments as sentences, and suppose we imagine the contextshiftingbetween the utterance of the premise and the utterance of the conclusion. Suppose you are hungry and utter the premise, and I am not hungry and utter the conclusion. Then we would have a truepremise and a falseconclusion, so the argument would not be valid. Clearly we need to avoid such problems, and introducing the notion of a proposition, in the style of this section, is one way of doing so.