Lyon, 17 May 1846:

Let us seek together, if you wish, the laws of society, the manner in which these laws are realized, the process by which we shall succeed in discovering them; but, for God’s sake, after having demolished all the a priori dogmatisms, do not let us in our turn dream of indoctrinating the people; do not let us fall into the contradiction of your compatriot Martin Luther, who, having overthrown Catholic theology, at once set about, with excommunication and anathema, the foundation of a Protestant theology. For the last three centuries Germany has been mainly occupied in undoing Luther’s shoddy work; do not let us leave humanity with a similar mess to clear up as a result of our efforts. I applaud with all my heart your thought of bringing all opinions to light; let us carry on a good and loyal polemic; let us give the world an example of learned and far-sighted tolerance, but let us not, merely because we are at the head of a movement, make ourselves the leaders of a new intolerance, let us not pose as the apostles of a new religion, even if it be the religion of logic, the religion of reason. Let us gather together and encourage all protests, let us brand all exclusiveness, all mysticism; let us never regard a question as exhausted, and when we have used our last argument, let us begin again, if need be, with eloquence and irony. On that condition, I will gladly enter your association. Otherwise — no!

Emphases mine. Edmund Wilson: “The result of this incident was that Marx was to set upon Proudhon’s new book with a ferocity entirely inconsonant with the opinion of the value of Proudhon’s earlier work which he had expressed and which he was to reiterate later” (To the Finland Station, p. 154).