 (This example is from Stanley Jevons, 1869. Boole’s own account is idiosyncratic, but I believe Jevons’ example represents Boole’s intentions accurately.) Today we would write \(\forall x(Px \rightarrow Qx)\) rather than \(P \subseteq Q\), but this is essentially the standard definition of \(P \subseteq Q\), so the difference between us and Boole is slight. 