This book examines the theory of science according to John Buridan (ca. 1295-1358). This author plays a major role in the 14th-century philosophy: not only is he a reference in all discussions of his time, but his style of philosophy also spread in Italy and in Central European universities. The medieval notion of scientia is multiple: mental disposition which gives justified assent to propositions (statements); organised discourse which must be analysed from the perspective of semantic criterias and argumentative modalities; encyclopaedia of profane or sacred disciplines. Buridan's work offers a chance to study all these dimensions. The book is divided into three parts. The first one deals with the science as disposition of mind. Scientificity is characterised by some of the mental act or disposition's features, before being due to objects on which they focus. Science and opinion, as well as the different sciences, adress to the same things, but consider them from different points of view and according different modalities. Consequently, the problem of science is centred on belief and its justifications. It is within this context that the ideas of universality and necessity must be redefined, in a contingent world composed of singular beings. The second part is a study of the language in which science is formulated, exposed and transmitted. It studies the terms and their referential properties, and the theory of demonstration which is identified with scientific reasoning since the rediscovery of the Posterior Analytics. At the same time, this second part shows how Buridan does not reduce all forms of knowledge to demonstrative knowledge and put emphasis on other forms, such as probable reasoning or prudential reasoning. Finally, the third part outlines the general system of knowledge. After having recalled the basic principles of ontological singularity and Buridan's treatment of the universal, this section outlines the division of sciences according to Buridan. Then, it details more particularly the case of metaphysics, mathematics and physics. Finally, it focuses on the relations between the sciences Buridan is talking about (disciplines that are purely philosophical, rational and human), and theology. The whole book does not only strengthen John Buridan's position as one of the greatest philosophers of the late Middle Ages, but it also highlights the emergence of major philosophical issues on the conception of science, on the problem of belief, evidence and certainty, on the value and nature of justifications, on the necessary and probable in the study of the common course of nature.
