The paper presents the main ideas of French conventionalism as represented by Duhem, Poincaré and Le Roy. Clarified are some misunderstandings and misinterpretations on which the negative opinion on conventionalism is usually based. Conventionalism is presented as a source of the antipositivistic breakthrough. It thus led to the most important discussions in 20th century philosophy of science, which tended to undermine epistemological fundamentalism. The influence of conventionalism is shown - on the European philosophy and specifically on the Polish one, especially on works of K. Ajdukiewicz and his followers.