Absorption (logic)

Absorption is a valid argument form and rule of inference of propositional logic.^[1][2] The rule states that if ${\displaystyle P}$ implies ${\displaystyle Q}$, then ${\displaystyle P}$ implies ${\displaystyle P}$ and ${\displaystyle Q}$. The rule makes it possible to introduce conjunctions to proofs. It is called the law of absorption because the term ${\displaystyle Q}$ is "absorbed" by the term ${\displaystyle P}$ in the consequent.^[3] The rule can be stated:

${\displaystyle \frac{P\to Q}{\therefore P\to (P\wedge Q)}}$

where the rule is that wherever an instance of "${\displaystyle P\to Q}$" appears on a line of a proof, "${\displaystyle P\to (P\wedge Q)}$" can be placed on a subsequent line.

The absorption rule may be expressed as a sequent:

${\displaystyle P\to Q⊢P\to (P\wedge Q)}$

where ${\displaystyle ⊢}$ is a metalogical symbol meaning that ${\displaystyle P\to (P\wedge Q)}$ is a syntactic consequence of ${\displaystyle (P\to Q)}$ in some logical system;

and expressed as a truth-functional tautology or theorem of propositional logic. The principle was stated as a theorem of propositional logic by Russell and Whitehead in Principia Mathematica as:

${\displaystyle (P\to Q)\leftrightarrow (P\to (P\wedge Q))}$

where ${\displaystyle P}$, and ${\displaystyle Q}$ are propositions expressed in some formal system.

If it will rain, then I will wear my coat.
Therefore, if it will rain then it will rain and I will wear my coat.

• Absorption law

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