Absorption (logic)

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

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

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

Formal notation

The absorption rule may be expressed as a sequent:

$P\to Q⊢P\to (P\wedge Q)$

where $⊢$ is a metalogical symbol meaning that $P\to (P\wedge Q)$ is a syntactic consequence of $(P\to Q)$ 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:

$(P\to Q)\leftrightarrow (P\to (P\wedge Q))$

where $P$, and $Q$ are propositions expressed in some formal system.

Examples

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.

Proof by truth table

$P$	$Q$	$P\to Q$	$P\to (P\wedge Q)$
T	T	T	T
T	F	F	F
F	T	T	T
F	F	T	T

Formal proof

Proposition	Derivation
$P\to Q$	Given
$\mathrm{\neg }P\vee Q$	Material implication
$\mathrm{\neg }P\vee P$	Law of Excluded Middle
$(\mathrm{\neg }P\vee P)\wedge (\mathrm{\neg }P\vee Q)$	Conjunction
$\mathrm{\neg }P\vee (P\wedge Q)$	Reverse Distribution
$P\to (P\wedge Q)$	Material implication

See also

• Absorption law