Law of thought

The laws of thought are fundamental logical rules, with a long tradition in the history of philosophy, which collectively prescribe how a rational mind must think. To break any of the laws of thought (for example, to contradict oneself) is to be irrational.Fact|date=July 2008


Socrates, in a Platonic dialogue, described three principles derived from introspection. He asserted that these three axioms contradict each other.Quotation| [F] irst , that nothing can become greater or less, either in number or magnitude, while remaining equal to itself … Secondly, that without addition or subtraction there is no increase or diminution of anything, but only equality … Thirdly, that what was not before cannot be afterwards, without becoming and having become.|Plato|Theatetus, 155

Indian logic

The law of noncontradiction is found in ancient Indian logic as a meta-rule in the "Shrauta Sutras", the grammar of Pāṇini, [citation|author=Frits Staal|title=Universals: Studies in Indian Logic and Linguistics|publisher=Chicago|year=1988|pages=109-28 (cf. citation|title=Seeing Things Hidden|first=Malcolm|last=Bull|publisher=Verso|year=1999|isbn=1859842631|page=53)] and the "Brahma Sutras" attributed to Vyasa. It was later elaborated on by medieval commentators such as Madhvacharya. [citation|title=A History of Indian Philosophy|first=Surendranath|last=Dasgupta|publisher=Motilal Banarsidass|year=1991|isbn=8120804155|page=110]


The three classic laws of thought are attributed to Aristotle and were foundational in scholastic logic. They are:

:* law of identity:* law of noncontradiction :* law of excluded middle


The classical Islamic philosopher, Ibn Sina (Avicenna), once wrote the following response to opponents of the law of noncontradiction:


John Locke claimed that the principles of identity and contradiction were general ideas and only occurred to people after considerable abstract, philosophical thought. He characterized the principle of identity as "Whatsoever is, is." The principle of contradiction was stated as "It is impossible for the same thing to be and not to be." To Locke, these were not innate or "a priori" principles.


Leibniz formulated two additional principles, either or both of which may sometimes be counted as a law of thought:

:* principle of sufficient reason :* identity of indiscernibles

In Leibniz's thought and generally in the approach of rationalism, the latter two principles are regarded as clear and incontestable axioms. They were widely recognized in European thought of the seventeenth, eighteenth, and (while subject to greater debate) nineteenth century. As turned out to be the case with another such (the so-called law of continuity), they involve matters which, in contemporary terms, are subject to much debate and analysis (respectively on determinism and extensionality). Leibniz's principles were particularly influential in German thought. In France the "Port-Royal Logic" was less swayed by them. Hegel quarrelled with the identity of indiscernibles in his "Science of Logic" (1812-1816).


Four Laws

Schopenhauer discussed the laws of thought and tried to demonstrate that they are the basis of reason. He listed them in the following way in his "On the Fourfold Root of the Principle of Sufficient Reason", §33:

#A subject is equal to the sum of its predicates, or a = a.
#No predicate can be simultaneously attributed and denied to a subject, or a = — a = 0.
#Of every two contradictorily opposite predicates one must belong to every subject.
#Truth is the reference of a judgment to something outside it as its sufficient reason or ground. Also:Quotation|The laws of thought can be "most intelligibly" expressed thus:
#Everything that is, exists.
#Nothing can simultaneously be and not be.
#Each and every thing either is or is not.
#Of everything that is, it can be found why it is.There would then have to be added only the fact that once for all in logic the question is about "what is thought" and hence about concepts and not about real things.|Schopenhauer, "Manuscript Remains", Vol. 4, "Pandectae II," §163

To show that they are the foundation of reason, he gave the following explanation:

Schopenhauer's four laws can be schematically presented in the following manner:
#A is A.
#A is not not-A.
#A is either A or not-A.
#If A then B.

Two Laws

Later, in 1844, Schopenhauer claimed that the four laws of thought could be reduced to two. "It seems to me," he wrote in the second volume of "The World as Will and Representation", Chapter 9, "that the doctrine of the laws of thought could be simplified by our setting up only two of them, namely, the law of the excluded middle, and that of sufficient reason or ground."Here is Law 1:

Law 2 is as follows:

He further asserted that "Insofar as a judgment satisfies the first law of thought, it is "thinkable"; insofar as it satisfies the second, it is "true" … ."


The title of George Boole's 1854 treatise on logic, "An investigation on the Laws of Thought", indicates an alternate path. The laws are now incorporated into his boolean logic in which the classic Aristotelian laws come down to saying there are two and only two truth values. The Leibnizian principles are ignored, at the algebraic level, absent second-order logic.


In the 19th century the Aristotelian, and sometimes the Leibnizian, laws of thought were standard material in logic textbooks, and J. Welton described them in this way:


Bertrand Russell discussed only the three classic Aristotelian laws of thought in his 1912 book "The Problems of Philosophy". At this point, in the early twentieth century, the laws of thought were sliding out of pedagogy in the field of logic, and the law of excluded middle was shortly to be questioned by intuitionistic logic.


* Aristotle, "The Categories", Harold P. Cooke (trans.), pp. 1-109 in "Aristotle, Vol. 1", Loeb Classical Library, William Heinemann, London, UK, 1938.
* Aristotle, "On Interpretation", Harold P. Cooke (trans.), pp. 111-179 in "Aristotle, Vol. 1", Loeb Classical Library, William Heinemann, London, UK, 1938.
* Aristotle, "Prior Analytics", Hugh Tredennick (trans.), pp. 181-531 in "Aristotle, Vol. 1", Loeb Classical Library, William Heinemann, London, UK, 1938.
* Boole, George, "An Investigation of the Laws of Thought on Which are Founded the Mathematical Theories of Logic and Probabilities", Macmillan, 1854. Reprinted with corrections, Dover Publications, New York, NY, 1958.
* Russell, Bertrand, "The Problems of Philosophy", Williams and Norgate, London, 1912.
* Arthur Schopenhauer, "The World as Will and Representation", Volume 2, Dover Publications, New York, 1966, ISBN 0-486-21762-0

ee also

* Laws of classical logic

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