 Just as Spinoza explains the matter but not the form of the physical world, so, too, Mendelssohn charges, he gives an account of the matter but not the form of the spiritual world. In this connection it is helpful to recall Mendelssohn’s differentiation of knowing, approving, and desiring from one another. Truth and falsity, corresponding to knowing, provide the matter of the spiritual world. While they may, as Spinoza maintains, have their origin in thought as the attribute of a single, infinite substance, categories corresponding not to knowledge but to approving or desiring must have some other source. Thus, according to Mendelssohn, Spinoza leaves unexplained “the difference between good and evil, desirable and undesirable, pleasure and pain” (Gesammelte Schriften, III/2, p. 109).