In the arena of ideas and debate and conversation, it is important to agree on definitions. If two people cannot agree on definitions and language, the conversation will be futile.
Even more general than definitions are agreements on things that are obvious. "If someone ... refuses to accept what is patently obvious, it is not easy to find arguments to use against him that could cause him to change his mind" (p. 13).
Worse still, are those who want to change meanings or words mid-conversation!
Epictetus rails against two types of obtuseness: that of the intellect and that of the moral compass.
If someone cannot intellectually carry on conversation, then you might as well begin talking to a brick.
And if someone lacks moral direction, Zeus help him and society!