It is customary to oppose knowledge to opinions. But what constitutes knowledge, and what is only opinion? What are the criteria for making such a division? The fact is that today we have at our disposal many criteria for making the division, and it is not difficult to enumerate them. To quote in random order, by way of example: observation, reliable evidence, rigorous application of method, the use of a precise language testing by experiment, power of anticipation, formalisation, ability to proceed to applications, usefulness of these applications, accountability to certain principles or to acquired knowledge, impartiality, accepted views of specialists, etc. Usually we bear in mind a few of these criteria and attempt to coordinate them; whereas others, which we do not consider indispensable or relevant, are put aside; this varies according to the knowledge envisaged … And one would be in greater difficulty trying to find a justification for such criteria! But this is not our purpose here. If we mention it, it is only in order to make it clear that we will not be concerned with such matters,nor with such question as when it is permissible to characterise this or that as "opinion" or "knowledge"? Our subject is different:we want to raise questions concerning the use which is made of this opposition between knowledge and opinions, concerning the ideology which underlines this opposition, and concerning the resultant effects for the sciences.