Supposing the concept causa sui absurd, the external world is not the work of our organs, because otherwise our body and then our organs themselves would be the work of our organs

The passage

Because, if the external world were the work of our organs, our body as a piece of it would be, and then our organs themselves would be, the work of our organs, and because, supposing the concept causa sui thoroughly absurd, our organs are not the work of our organs (the map's bridge, which reads organs that are their own work as a case of causa sui), the external world is, under that supposition, not the work of our organs.

The argument

1. P1If the external world were the work of our organs, our body, as a piece of it, would be the work of our organs
2. P2If our body were the work of our organs, our organs themselves would be the work of our organs
3. P3Assumption. If the concept causa sui is something thoroughly absurd, then our organs are not the work of our organs (the map's bridge: the text calls the result »eine gründliche reductio ad absurdum: gesetzt, dass der Begriff causa sui etwas gründlich Absurdes ist«, and the map reads organs that are the work of our organs as a case of causa sui, which the supposition excludes)
4. Infer.hypothetical syllogism and contraposition
5. CSupposing the concept causa sui absurd, the external world is not the work of our organs, a sentence the text closes with a question mark

The argument is the second half of JGB 15, which the text itself calls »eine gründliche reductio ad absurdum«: »Aber dann wäre ja unser Leib, als ein Stück dieser Aussenwelt, das Werk unsrer Organe! Aber dann wären ja unsre Organe selbst - das Werk unsrer Organe! Dies ist, wie mir scheint, eine gründliche reductio ad absurdum: gesetzt, dass der Begriff causa sui etwas gründlich Absurdes ist. Folglich ist die Aussenwelt nicht das Werk unsrer Organe -?«

What the form captures and what it leaves out. Premises 1 and 2 are the two »dann« sentences, each a conditional whose antecedent is the consequent of the step before, the first grounded in the text (»als ein Stück dieser Aussenwelt«), the second not. Premise 3 is the map's bridge: the text does not say that organs which are the work of our organs are a causa sui, nor that what is absurd does not obtain; it calls the chain a reductio ad absurdum and names the supposition under which it is one, and the map reads the two together as the conditional that, if the concept causa sui is thoroughly absurd, our organs are not their own work. A reader may dispute that reading, or hold that the text's absurdity is not the causa sui but something else in the chain. The conclusion is the conditional the text's »gesetzt« makes it, from the supposition to the denial; its antecedent is the supposition atom for atom, and the form proves no more than the claim states. The text's closing »-?« is kept in the conclusion claim's statement and on its passage, and the form does not represent it: a truth table has no mark for a conclusion put as a question. The form does not cover the first sentence of the aphorism, on the sense organs (c-nietzsche-sense-organs-not-appearances-in-idealist-sense), which gives no premise to the chain, nor the »Andere« whose view the chain answers, who are not named. JGB 21 asserts in Nietzsche's own voice that the causa sui is a self-contradiction (c-nietzsche-causa-sui-best-self-contradiction); the argument does not take it as a premise, since JGB 15 supposes and does not assert, and the conclusion would then lose the supposition the text gives it.