the 1870s, and he agrees in principle with this derivation of the principle of contradiction (Logik, vol. 1, §45, p. 5 n.). He calls the principle of contradiction a law of nature that says, “it is impossible at any given moment to say, with conscious awareness, that A is B and that A is not B.” Because it is a law of nature, it can also be understood as a standard law aimed at the practical regulation of thinking. In other words, it “applies to the whole range of constant concepts.”11
It not only works as but also is a lawful demand, and it also presents itself as a norm for the procedure of thinking.
The validity of the principle rests “on the immediate awareness that, in negation, we always do and always will do the same thing—so certainly we are the same persons” (Sigwart, Logik, p. 402).12
In this way the lawfulness—and consequently, the validity—of thinking are reduced to the uniformity in the constitution of our nature [41] and of our way of thinking. Thus Sigwart:
On the other hand, if we deny the possibility of knowing something as it is in itself, if the entity is only a thought that we produce {Sigwart thinks he is reporting the Kantian position here}, then it holds that we even at-tribute objectivity to such ideas that we produce with the consciousness of their necessity. And it holds that as soon as we posit something as existing, we consequently affirm that (even if taken hypothetically) all other think-ing beings that have the same nature as ours have to produce the same thoughts with the same necessity. (Logik, §1, p. 8)
So even if we admit that, to put it roughly, we cannot know the outer world as it is but can only regulate and order our representations, nonetheless the necessity with which we connect certain representations with one another is the criterion for the objective validity of whatever we think by means of these representations. And we make the presupposition that other [thinking] beings are organized the way we are, and that they have to think what we think. Therefore, a communally held knowledge of the objective world is possible in spite of the fact that we do not get outside our own consciousness. In the same place Sigwart also says that this necessary and universally valid thinking is nothing else but the concept of what we call the essence of truth. Truth is nothing but the necessity and universal validity of the combination of representations, a necessity that is ultimately ruled by the principle of contradiction, the validity of which is founded on our mental nature.
Lipps says,
11. [H. Sigwert, Logik, 4th edition (1921), p. 401.]
12. Validity is reduced to the constancy of our behavior and our being; we are firmly predisposed in such a way that we cannot do otherwise.