Absorption (logic)

Absorption is a valid argument form and rule of inference of propositional logic.^[1]^[2] 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.^[3] The rule can be stated:

{\frac {P\to Q}{\therefore P\to (P\land Q)}}

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

Formal notation

The absorption rule may be expressed as a sequent:

P\to Q\vdash P\to (P\land Q)

where $\vdash$ is a metalogical symbol meaning that $P\to (P\land Q)$ is a syntactic consequence of $(P\rightarrow Q)$ 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\land Q))

where $P$ , and $Q$ 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.

$P$	$Q$	$P\rightarrow Q$	$P\rightarrow (P\land Q)$
T	T	T	T
T	F	F	F
F	T	T	T
F	F	T	T

^ Copi, Irving M.; Cohen, Carl (2005). Introduction to Logic. Prentice Hall. p. 362.
^ "Rules of Inference".
^ Whitehead and Russell, Principia Mathematica, p. 14.