The fundamental error of all systems is the failure to understand this truth. _Intelligence and matter are correlates_, _i.e._, the one exists only for the other, both stand and fall together, the one is only the reflex of the other. Indeed they are really _one and the same thing_ regarded from two opposite points of view; and this one thing, I am here anticipating, is the manifestation of the will, or the thing in itself. Consequently both are secondary, and therefore the origin of the world is not to be sought in either of the two. But because of their failure to understand this, all systems (with the exception perhaps of that of Spinoza) sought the origin of all things in one of these two. Some of them, on the one hand, suppose an intelligence, νους, as the absolutely First and δημιουργος, and accordingly in this allow an idea of things and of the world to precede their actual existence; consequently they distinguish the real world from the world of idea; which is false. Therefore matter now appears as that through which the two are distinguished, as the thing in itself. Hence arises the difficulty of procuring this matter, the ὑλη, so that when added to the mere idea of the world it may impart reality to it. That original intelligence must now either find it ready to hand, in which case it is just as much an absolute First as that intelligence itself, and we have then two absolute Firsts, the δημιουργος and the ὑλη; or the absolute intelligence must create this matter out of nothing, an assumption which our understanding refuses to make, for it is only capable of comprehending changes in matter, and not that matter itself should come into being or pass away. This rests ultimately upon the fact that matter is essential, the correlate of the understanding. On the other hand, the systems opposed to these, which make the other of the two correlates, that is, matter, the absolute First, suppose a matter which would exist without being perceived; and it has been made sufficiently clear by all that has been said above that this is a direct contradiction, for by the existence of matter we always mean simply its being perceived. But here they encounter the difficulty of bringing to this matter, which alone is their absolute First, the intelligence which is finally to experience it. I have shown this weak side of materialism in § 7 of the first volume. For me, on the contrary, matter and intelligence are inseparable correlates, which exist only for each other, and therefore merely relatively. Matter is the idea of the intelligence; the intelligence is that in whose idea alone matter exists. The two together constitute _the world as idea_, which is just Kant’s _phenomenon_, and consequently something secondary. What is primary is that which manifests itself, _the thing in itself_, which we shall afterwards discover is the will. This is in itself neither the perceiver nor the perceived, but is entirely different from the mode of its manifestation.