The main purpose of this paper is to explore the reasons Aristotle gives for being able to use rhetorical argumentation, which is obviously not a scientific mode of expression. This faculty which was condemned by Plato as lacking morality, is paradoxically regarded by Aristotle as necessary on moral grounds. For, according to him, it would be blameworthy to keep silent when being verbally assailed. The necessity of rhetoric is, however, more deeply founded. First, because justice has to be saved from its enemies in the City's courts of law. Secondly, because everyone has to be convinced to follow in practice the rules of the City's laws and such a conviction, in the case of the multitude, cannot be obtained by the means of scientific arguments. Thirdly and above all, because, in forensic disputes, characteristic of free political societies, the demagogic power, which regularly leads to tyrannical regimes, can only be avoided by the weapons of rhetoric. From these explanations, one does see that rhetoric, for Aristotle, seems to be the necessary substitute for ancient and traditional instruments securing obedience to legal justice, i.e. myths and pure constraint or coercion. In civilized and free political communities, rhetoric is required for civilization and freedom.