Laws of Thought

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As laws of thought were in the history of philosophy and philosophical logic , especially in the psychologism of the 19th century, called logical rules, laws or principles in that they - this was the psychologistic view - as natural laws of thought were considered.

Gottlob Frege remarked on the relationship between psychology and logic :

“It is generally admitted in advance that the logical laws should be guidelines for thinking in order to reach the truth; but it is all too easily forgotten. The double meaning of the word "law" is fatal here. In one sense it says what is, in the other it prescribes what should be. It is only in this sense that logical laws can be called laws of thought, in that they establish how one should think. Every law that says what is can be understood as prescribing, it should be thought in accordance with it, and is therefore a law of thought in that sense. This is no less true of the geometric and physical than of the logical. These only deserve the name “Laws of Thought” if it is meant to say that they are the most general ones, which prescribe everywhere how to think, wherever there is thought at all. But the word “law of thought” leads to the opinion that these laws govern thinking in the same way as the laws of nature govern the processes in the outside world. Then they cannot be anything other than psychological laws; because thinking is a mental process. And if logic had to do with these psychological laws, it would be a part of psychology. And so it is, in fact, conceived. As a guideline, these laws of thought can then be interpreted in such a way that they give a mean average, similar to how one can say how healthy digestion goes on in humans, or how to speak grammatically correct, or how to dress modern. One can then only say: according to these laws, on average, people are held to be true, now and as far as people are known; so if you want to stay in tune with the average, go with them. But, just as what is modern today will no longer be modern after a while and is not modern among the Chinese, so one can only make the psychological laws of thought authoritative with reservations. Yes, if logic was about being held to be true, and not rather about being true! And the psychological logicians confuse that. "

In particular, different sentences of identity, the sentence of contradiction, the sentence of excluded third and the sentence of sufficient reason were combined into a group with the terms laws of thought and logical principles . These propositions, which exist in different formulations, have in the tradition been viewed partly as logical , partly as metaphysical and partly as epistemological principles and as such have been both defended and contested.

Theorem of identity (Latin principium identitatis)
On Aristotle the principle of self-identity of all things, i.e. H. the statement A = A valid for every A is returned. The principle of the identity of indistinguishable things goes back to Leibniz , a metaphysical principle according to which the following applies to things from a discourse universe : If A and B are qualitatively identical (i.e. if they have exactly the same properties) they are also numerically identical (A = B).
Theorem of contradiction (lat. Principium contradictionis)
Going back to Aristotle, the principle of contradiction says that it is impossible to both affirm and deny a statement at the same time.
Theorem about the excluded third party (Latin principium exclusi tertii)
Also traced back to Aristotle, the proposition of the excluded third means that the disjunction of a statement and its negation is always a valid statement, i.e. a tautology . This theorem is related, but not identical, to the principle of bivalent . A logic that follows the principle of the excluded third and in which inferences that correspond to the disjunctive syllogism are allowed is necessarily bivalent.
Theorem of sufficient reason (Latin principium rationis sufficientis)
A logical-metaphysical principle from Gottfried Wilhelm Leibniz , which states that every event must have a cause or that for every true statement there is a reason why it is true. Interpreted as an instruction to act, the principle of sufficient reason requires that every true statement is justified by another statement whose truth has been proven. The possible violations of this instruction are called circular reasoning and petitio principii .

literature

  • Hartley Slater: Law of Thought, In: Polimetrica Onlus (ed.): “The Language of Science”, ISSN  1971-1352 .

See also

Individual evidence

  1. ↑ Gottlob Frege: Fundamental laws of arithmetic. Volume 1, 2nd edition Jena 1903, reprint: Olms, Braunschweig 1998, p. XV, online at korpora.org .