Schopenhauer wrote:In actuality, the sought-for meaning of the world which confront me solely as my mental picture, or the transition from this world, as mere mental picture of the subject knowing it, over to what it might still be besides mental picture, could nevermore be found, if the researcher himself were nothing more than purely knowing subject (winged angel's head without body). But now he himself has roots in that world, finds himself in it, namely, as an individual, which means that this activity of knowing, which is the determining bearer of the whole world as a mental picture, is after all given entirely through the medium of a body, whose sensations, as shown, are the starting point for the intellect in viewing the world. For the purely knowing subject as such, this body is a mental picture like any other, an object among objects: the motions, the actions of it are known to him in that respect no differently than the changes in all other observable objects, and would be just as foreign and incomprehensible to him, if the significance of his own motions and actions were not disclosed to him somehow in a completely different way.
... To the knowing subject, which arises as an individual through its identification with the body, this body is given in two completely different ways: one is as a mental picture when the body is viewed intellectually, as object among objects, and subject to the laws of these objects but then at the same time in a completely different way also as that something, known directly by everyone, which the word “will” characterizes. Every true act of his will is immediately and unfailingly also a movement of his body; he cannot really will an act, without at the same time perceiving that it manifests as a movement of his body. The act of will and the action of the body are not two objectively known different states, connected by the bond of causality; they do not stand in the relationship of cause and effect; but they are rather one and the same, only given in two completely different ways: one completely direct and one for the intellect in contemplation.
I have tried to use emphasis here so people can follow along easier, although I think it's pretty clear anyway. The bolded assertions = what Schop believed about thinking activity, and the underlined assertions = what he believed about willing activity. It should very clear that the former, in Schop's view, is completely personal (individual) and provides "mere mental pictures". And the latter, as BK also argues, is what truly bears the world's Unity in a "completely direct" way. Thinking (illusory) discloses things for Schopenhauer in a completely different way than Willing (non-illusory), and the latter is what provides any true knowledge of the noumenal world, while the former is mere intellect which is an "object among objects". This next part is small portion of Steiner's response to Schop (which is really spread throughout the entire book), but it should reveal clearly that Steiner understands Schop's argument.
Steiner wrote:By this train of thought Schopenhauer believe himself justified in finding the objectivity of will within the human body. He is of the opinion that, in the actions of the body, he feels directly a reality, the thing-in-itself in concrete. Against these arguments it must be objected that the actions of our body come to consciousness only through self-perceptions and as such have nothing over other perceptions. If we want to know their nature, we can do this only through thinking contemplation, that means through incorporating them into the ideal system of our concepts and ideas.