If there is a robustness of science and knowledge, which makes their cumulative dimension possible, they can also be weakened. We shall discuss here two apparently quite different cases of relativization of knowledge: "agnotology" or so-called strategic ignorance on the one hand, and "post-truth" on the other. Our aim is first to explain and illustrate their history and manifestations. We will then try to show what links them in their causes and will evoke how to fight them.