In logic, a contradiction consists of a logical incompatibility between two or more propositions. It occurs when the propositions, taken together, yield two conclusions which form the logical inversions of each other. Illustrating a general tendency in applied logic, Aristotle’s law of noncontradiction states that “One cannot say of something that it is and that it is not in the same respect and at the same time.”

By extension, outside of formal logic, one can speak of contradictions between actions when one presumes that their motives contradict each other.

In formal logic, particularly in propositional and first-order logic, a proposition ${\displaystyle \varphi }$ is a contradiction if and only if ${\displaystyle \varphi \vdash \bot }$. Since for contradictory ${\displaystyle \varphi }$ it is true that ${\displaystyle \vdash \varphi \rightarrow \psi }$ for all ${\displaystyle \psi }$ (because ${\displaystyle \varphi \rightarrow \bot \rightarrow \psi }$), one may prove any proposition from a set of axioms which contains contradictions.

For a proposition ${\displaystyle \varphi }$ it is true that ${\displaystyle \vdash \varphi }$, i. e. that ${\displaystyle \varphi }$ is a tautology, i. e. that it is always true, if and only if ${\displaystyle \neg \varphi \vdash \bot }$, i. e. if the negation of ${\displaystyle \varphi }$ is a contradiction. Therefore, a proof that ${\displaystyle \neg \varphi \vdash \bot }$ also proves that ${\displaystyle \varphi }$ is true. The use of this fact constitutes the technique of the proof by contradiction, which mathematicians use extensively. This applies only in a logic using the excluded middle ${\displaystyle A\vee \neg A}$ as an axiom.

### Symbolic representation

In mathematics, the symbol used to represent a contradiction within a proof varies. [1] Some symbols that may be used to represent a contradiction include ↯, ⇒⇐ , ⊥, ↮, and ※. It is not uncommon to see Q.E.D. or some variant immediately after a contradiction symbol; this occurs in a proof by contradiction, to indicate that the original assumption was false and that the theorem must therefore be true.

Adherents of the epistemological theory of coherentism typically claim that as a necessary condition of the justification of a belief, that belief must form a part of a logically non-contradictory (consistent) system of beliefs. Some dialetheists, including Graham Priest, have argued that coherence may not require consistency.

### Self-refuting statements and performative contradictions

It often occurs in philosophy that the content or presence of the argument contradicts the claims of the argument; for example: Heraclitus’s proposition that knowledge is impossible; or, arguably, Nietzsche’s statement that one should not obey others. These are self-refuting statements and performative contradictions.

### In colloquial speech

Colloquial usage can label actions or statements (or both) as contradicting each other when due (or perceived as due) to presuppositions which are contradictory in the logical sense.

### In dialectics

#### Marxism

In dialectical materialism, contradiction, as derived by Karl Marx from Hegelianism, usually refers to an opposition of social forces. Most prominently (according to Marx), capitalism entails a social system that has contradictions because the social classes have conflicting collective goals. These contradictions stem from the social structure of society and inherently lead to class conflict, economic crisis, and eventually revolution, the existing order’s overthrow and the formerly oppressed classes’ ascension to political power.[citation needed]

Mao Zedong's most important philosophical essay furthered Marx and Lenin's thesis and suggested that all existence is the result of contradiction.