  Many people have wanted to give an account of logical validity which pays some attention to conditions of relevance. If \(X,A \vdash B\) holds, then \(X\) must somehow be relevant to \(A\).  Premise combination is restricted in the following way. We may have \(X \vdash A\) without also having \(X,Y \vdash A\) . The new material \(Y\) might not be relevant to the deduction. In the 1950s, Moh (1950), Church (1951) and Ackermann (1956) all gave accounts of what a ‘relevant’ logic could be. The ideas have been developed by a stream of workers centred around Anderson and Belnap, their students Dunn and Meyer, and many others.  The canonical references for the area are Anderson, Belnap and Dunn’s two-volume Entailment (1975 and 1992). Other introductions can be found in Read’s Relevant Logic, Dunn and Restall’s Relevance Logic (2002), and Mares’ Relevant Logic: a philosophical interpretation (2004). 