The paper focuses on a basic principle of Aristotle’s Rhetoric according to which the more a discourse is able to produce in the hearer a quick and pleasant learning, the more it will be persuasive. Starting from the numerous examples provided by Aristotle, the paper shows that this principle, which we can call Pleasure and Knowledge Principle (PKP), regards all the levels of a speech (inventio as well as elocutio, actio, and dispositio) and it is founded in Aristotle’s anthropology and epistemology. Thanks to this feature, PKP can be considered the common thread of the whole Rhetoric as well as a good starting point for a general reflection on the persuasive power of language and its role in human life. More exactly, PKP can help us to ove...