Retrieving an insight dating back to antiquity, this essay argues that the confrontation of opposing views and arguments is desirable in political deliberation. But freedom of speech and diversity among deliberators do not suffice to secure that outcome. Therefore we should actively facilitate and encourage the presentation of contrary opinions during deliberation. Such confrontation is our best means of improving the quality of collective decisions. It also counteracts the pernicious fragmentation of the public sphere. It facilitates the comprehension of choices. Lastly, arguing for and against a given decision treats the minority with respect. This essay proposes practical ways of promoting adversarial deliberation, in particular the organization of debates disconnected from electoral competition.