 RTT+COMP+CCTM gives a compelling explanation. During an inference from p and q to p, you transit from believing* mental sentence \(S_1 \amp S_2\) (which means that p and q) to believing* mental sentence \(S_{1}\) (which means that p). According to CCTM, the transition involves symbol manipulation. A mechanical operation detaches the conjunct \(S_{1}\) from the conjunction \(S_1 \amp S_2\). The same mechanical operation is applicable to a conjunction \(S_{3} \amp S_{4}\) (which means that m and n), corresponding to the inference from m and n to n. An ability to execute the first inference entails an ability to execute the second, because drawing the inference in either case corresponds to executing a single uniform mechanical operation. More generally, logical inference deploys mechanical operations over structured symbols, and the mechanical operation corresponding to a given inference pattern (e.g., conjunction introduction, disjunction elimination, etc.) is applicable to any premises with the right logical structure. The uniform applicability of a single mechanical operation across diverse symbols explains inferential systematicity. Fodor and Pylyshyn (1988) conclude that inferential systematicity provides reason to accept RTT+COMP+CCTM.