Logic is a psychological discipline precisely because knowing occurs only in the mind, and because thinking, which reaches its completion in know-ing, is a mental happening.6
Recall as well that in the nineteenth century the exact scientific method of modern natural science was carried over into psychology. According to its proponents, only in this way did psychology work itself up to the level of an exact science. That implies that, along with this psychology, there was also founded for the first time the exact and strict science that, by exact investigations into thinking and its laws, was also sure to create the exact and strict foundations of logic.7
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In this view, the inclusion of logic within psychology is clear and selfevident, and the reasoning that argues for this connection is so consistent that scarcely an objection can be raised against it. And so [39] this interpretation of the meaning of logic and the project of psychology as regards logic gained widespread acceptance.
As the art of correct thinking, logic will primarily aim at the securing of correctness and at conformity to rules and laws. Its basic theme is the conformity of thought-processes to law, the lawfulness of thinking, which from time immemorial has been formulated as basic laws of thinking. If these laws are to rule all scientific thinking, they themselves cannot rest on insecure ground but must be proven and demonstrated with the highest degree of scientific certitude. They cannot be invented but must be gotten from the data of thought itself, and their universal validity must be demonstrated.
Psychologism reveals its characteristic way of asking questions in the very way it handles and interprets the principles of thought. As an illustration we choose its interpretation of the principle of contradiction and its way of proving the law-giving nature of this principle. We will do so in a general way, without going into the specific content of its structure and the full determination of its proper meaning. The principle: “The same proposition cannot at the same time be both true and false.”
John Stuart Mill writes:
I consider it {the principium contradictionis} to be, like other axioms, one of our first and most familiar generalizations from experience. The original
6. [Lipps, op. cit.; cited in Husserl, LU, vol. 1, §18, p. 52 n. 2 / tr. 91 n. 1.]7. [Here (Moser, p. 79) Heidegger ends his lecture of Wednesday, 11 November 1925 (also the seventh anniversary of the end of the Great War), to be followed by that of Thursday, 12 November, which opened with a 420-word summary that is omitted in GA 21.]