Kant's derivation of the thing in itself by the law of causality cannot bring one in, because that law never leads beyond representation

The passage

Because Kant grounds the presupposition of the thing in itself on an inference by the law of causality from sensation to an external cause, and the law of causality holds only for representation and can never lead beyond it, that inference cannot bring in a thing in itself, which would be something wholly different from and independent of representation, nor show one to be a necessary presupposition.

The argument

1. P1Kant grounds the presupposition of the thing in itself on an inference by the law of causality from sensation to an external cause
2. P2The law of causality holds only for representation and can never lead beyond it
3. P3Assumption. If the presupposition of the thing in itself rests on an inference by the law of causality, and that law can never lead beyond representation, then the inference brings in nothing wholly different from and independent of representation, which is what a thing in itself would be, nor shows one to be a necessary presupposition (the map's bridge: the definition of a thing in itself the paragraph uses, ZA II 535, »nichts von ihr gänzlich Verschiedenes, von ihr Unabhängiges«, matching § 21's »toto genere von ihr verschieden«, ZA I 154)
4. Infer.modus ponens
5. CAn inference by the law of causality cannot bring a thing in itself into empirical intuition, which remains a mere process in us

The argument is the paragraph of the appendix that begins »Mit der in der ersten Auflage« (ZA II 534–535), which Schopenhauer says »läßt sich mit sehr Wenigem deutlich machen«: Kant's thing in itself rests on a causal inference from sensation to an external cause; the law, the sensation and the space in which the cause is placed are each subjective (c-schopenhauer-causality-sensation-and-space-are-subjective); »Mithin« the whole empirical intuition stays »auf subjektivem Grund und Boden, als ein bloßer Vorgang in uns«, and nothing »von ihr gänzlich Verschiedenes, von ihr Unabhängiges« can be brought in as a thing in itself or shown to be a necessary presupposition.

What the form captures and what it leaves out. Premise 1 is the first sentence of the exposition. Premise 2 is not stated in the appendix paragraph: the map reads the »Mithin« as resting on the principle § 19 states in general, that the law of causality holds only for representation »und über welches hinaus es nie führen kann« (ZA I 146–147), and lists that passage beside the appendix for that reason; what the appendix itself gives in its place is the subjectivity of the law, the sensation and the space, which the map records as a claim and does not put in the form, since the step from the three subjective items to »Mithin« is the step premise 2 states. Premise 3 is the map's bridge and is near to a definition: a thing in itself is what is wholly different from and independent of representation, so an inference whose law never leads beyond representation brings in none. The form does not touch whether premise 1 is true of Kant: Schopenhauer gives no passage for it and says the inference is »unter mancherlei Wendungen verdeckt«. The map sets no attack link on a Kant node, because no node of Kant's in the map states the inference Schopenhauer attributes to him, and because Schopenhauer accepts what Kant's nearest claims say, that a thing in itself underlies appearance (c-schopenhauer-only-the-derivation-is-faulty); the Kant passages in the map that speak of the senses being affected are Prolegomena § 13 Anm. II (AA 4:289, »indem sie unsere Sinne afficiren«) and § 32 (AA 4:314.36–315.01). The last sentence of the paragraph, the positive route through self-consciousness, is not part of the argument (c-schopenhauer-thing-in-itself-reached-only-through-self-consciousness).