Celebrated in his time for a few ponderous works on physics but now practically forgotten, the Dutch scientist Nicolaas Hartsoeker was one of the correspondents with whom Leibniz discussed atomistic philosophy. This essay analyzes the main aspects of their exchange of letters, focusing in particular on the problem of ‘hardness’. While Hartsoeker favors a voluntaristic account, attributing the qualities of matter directly to God’s will, Leibniz looks for an explanation in terms of matter and motion in accordance with mechanical philosophy. The solution Leibniz proposes resorts to the notion of motus conspirans (influenced in part by Hobbes), that is to common motion, which is internal to all solid bodies. He compares this mechanism to "fleuves ou jets" flowing inside the body and endowing it with a consistency which explains cohesion and hardness. The final part of the essay mentions a few problems with this theory, which after Leibniz was universally rejected, in particular by Newton.
Keywords: Leibniz, Hartsoeker, Hobbes, atomism, cohesion, hardness, theory of matter, conspiring motion