  In the chapter Of Expression and Interpretation, Boole said “the class not-X will be determined by  the symbol \(1-x\)”.  This is the first appearance of subtraction in MAL.  Boole’s initial equational expressions of the Aristotelian categorical propositions  (MAL, pp. 21,22) will be called his primary expressions. Then in the next several pages he adds supplementary expressions; of these the main ones will be called the secondary expressions.