Living in a democracy or working in a group requires the use of deliberative processes to agree and decide on ways of living together and projecting ourselves into common desirable futures. However, these processes remain an illusion, according to the political philosopher Chantal Mouffe. Because, decision by consensus often marginalises minority opinions, but also, rationality does not make it possible to overcome conflicts. They are rather often rooted in affects. Consequently, how can we open spaces for debate that are participatory, inclusive and that mobilise the affects? What methods and roles for such an agnostic design (from the Greek Agon, adversary)? My first contribution is the definition of the group of practices and of the research field of design for debate. Among these practices, my study focuses on “Discursive Design” for debate, in which programmes such as Critical Design, Speculative Design and Design Fiction participate. The fieldwork (5 design projects) revealed how design can stimulate interpersonal debate when it generates a ‘dissonance’ among the social values of the public, by presenting an ambivalent artefact (which juxtaposes discordant values). I have called this form of ethnomethodology through design, the bridging experiment. As a second result, beyond the simple design of an artefact, design can reach and mobilise a “public” (in the sense of John Dewey) concerned by a latent issue, by joining it in its own context. And, by orchestrating a whole communication situation where audiences and artefacts meet. I offer a descriptive model called the Discursive Design Communication System. Thus, when it thwarts the polarisation of opinions, the artefact takes on the role of a non-­human diplomat, which intensifies conflicts in order to connect worlds that do not speak to each other. But also, as a media, design adopts the role of an “agnostic mediating artefact,” which opens up multidimensional communication situations—between human, non-­human and fictional actors.