 Mendelssohn acknowledges that, despite the fact that the method is the same and the content in each case (mathematics and metaphysics) is an intrinsic character of things, progress in metaphysics has lagged far behind that in mathematics. He suggests three “objective” reasons for this lag: first, metaphysics’ greater reliance upon arbitrary signs (signs that do not essentially coincide with what is signified); second, the holistic content of metaphysics (no quality can be defined without an adequate insight into the others); and third, metaphysics’ need to establish the actual existence of what corresponds to the analyzed concepts. Thus, for Mendelssohn, mathematical truths need not suppose more than the appearances of things as long as a distinction is maintained between constant and inconstant appearances or, equivalently, between appearances that have their basis in the intrinsic, essential constitution of our senses and those that do not (e.g., those due to sickness or a faulty perspective). “Thus, even in the system of a doubter or an idealist, the value of not only pure, theoretical mathematics but even practical and applied mathematics remains, and it retains its undeniable certainty” (Philosophical Writings, p. 268). Metaphysics, by contrast, requires a resolution of the problem of idealism.