A statement should be self-consistent both in its expression and the assertion it conveys

This is a version of a very fundamental logical principle first stated by Aristotle. It is sometimes called, somewhat confusingly, the 'law of contradiction' and is expressed in various ways. For example, one version runs 'a class of entities cannot both have a certain property and not have it' or more abstractly: 'A cannot be both B and not-B'. A case of these versions would be, for example, 'A guitar cannot be both a stringed instrument and not a stringed instrument'. Whether this law of contradiction holds true for absolutely all individual things or all classes of entity is problematical and belongs more to the sphere of logic and ontology than semantics, though these are frequently interrelated. In the above formulation as the principle of non-contradiction the accent lies on language and the assertions made by it rather than on ontological or scientific questions as to the nature of actual things and events. As a principle in semantics it sets a norm for clear thinking and expression rather than expressing any law about the nature of things. The aim is to remove the self-contradictory use of terms and the concepts they express.

More explicitly, the principle of non-contradiction is broken in the following typical cases:-

Case 1) Where the same term is used twice in opposite senses

Eg) 'The colour white is not a colour'. This is a case of circularity of meaning or of a 'vicious circle in thought'. The tern 'colour' is used in two incompatible senses, for white cannot be both a colour and not a colour. We might assume that the intended assertion was badly expressed and would have been self-consistent in the expression "White is not a colour". Though white is often regarded as and called a colour in ordinary usage or in Goethe's theory of colour, Newton's theory requires that white is the neutral, uncoloured quality of light, the refraction of which alone produces colours. We are not here concerned with the ontological or scientific question of what whiteness itself is, only with being consistent in the use of the term 'white'.

Obviously the same term can be used in opposite senses in different expressions, such as in different chapters of a book. This would extend the principle beyond its present formulation. It would be valid and useful in application if extended to the case of any two or more statements because it is in long texts that it is most likely to be broken and go unnoticed.

Case 2) Where the logical predicate of the assertion expressed contradicts the logical subject

Eg) "We know for certain that nothing can be known for certain."

Let us presume that the terms 'know' and 'certain' have the same sense in the example, then the assertion itself is absurd. The logical subject 'We know for certain' is contradicted by the logical predicate 'that nothing can be known for certain' because the term 'nothing' excludes the possibility of knowing anything for certain. Since this leads thought into a vicious circle, it cannot effectively communicate any meaningful assertion. This indicates the relevance of this principle to semantics.

Eg. "Sixty-five plus twenty-one equals ninety-six".

According to the axioms of mathematical usage of numerical terms, this is a contradiction. The assertion expressed is not clearly conceivable to the mind, so we reject it as an error, whether as one of calculation (conceptual confusion) or of a misuse of terms.

With this last example in mind we see that the principle aims at ensuring that assertions are made clearly conceivable as a prerequisite of efficient communication. Care should be taken in analysing for suspected breaks of the principle because apparent contradictions may prove solvable on closer inspection. For example, "Let us assume that humans cannot attain certain knowledge" is not contradictory because 'Let us assume that' makes the statement conditional, not a categorical assertion of the truth.

Case 3) Where a term or expression is used in a way that contradicts common usage, without making clear that it is to be used in this uncommon way.

Eg) "What is totally impossible can only be achieved by aiming to do what is impossible".
The common usage of the term totally impossible' is quite categorical and a directly contrary term to 'possible'. 'Impossible' commonly includes 'that which cannot be achieved'. The above example therefore expresses a self-contradictory assertion.


1) Apply the principle of non-contradiction to the following expressions! "At 10 p.m. on January 10th 1982 it was raining at Blindern, Oslo."
"At 10 p.m. on January 10th 1982 the sun was shining at Blindern, Oslo."
Is there a contradiction in terms between the two expressions?

2) Analyse the following advertisement texts to decide whether each one is self-contradictory or not, giving reasons:-
"Munchables are simply the best, while our new Super-munchables are even better."
"You can't go wrong with our latest Cursill washing powder, and if you do then there's Extra Cursill, which any fool can use properly."

3) Decide whether the following expression is contradictory or not, explaining upon what you base your decision:-
"The balance of power that all agree presently exists between the Warsaw Pact countries and those of N.A.T.O. depends entirely upon the maintenance of the U.S.A.'s military superiority over the U.S.S.R."

4) EO "Nothing is ever created or destroyed, for what exists in creation always arises from or is destroyed by some creator."
a) Consider whether contradiction arises in the above expression and, if so, whether you judge it to be the result of inconsistent use

5) Consider the following exchange of views:-
A: The advance of micro-computer technology will result in less employment.
B: Yes. It can result in shorter working hours for all.
A: But it will also result in fewer jobs than will be needed.
B: I don't necessarily agree, for that depends entirely upon how it is introduced into economic life.
a) State clearly A's and B's interpretations of the term 'less employment' in A's first expression.
b) Evaluate whether these interpretations are more-precise than the, term 'less employment', giving grounds,
c) Does pseudo-agreement occur in the above discussion? Give grounds.