  Disjunctive propositions are used whenever an abstract term is ‘developed’ in its constituent parts or subclasses – whenever the extent of meaning of a term is explored. In order to represent disjunctive propositions, Jevons suggests to use the symbol ‘·|·’. The words ‘and’ and ‘or’, used in everyday language, can denote exclusive or inexclusive alternatives, but Jevons' symbol ‘·|·’ refers to alternatives that are not exclusive. The law of unity, A ·|· A = A, reveals an imperfect analogy between mathematics and logic. In his earliest work on logic Jevons used the symbol ‘+’ instead of ‘·|·’, but in The Principles of Science he recognises that the analogy between logic and mathematics is imperfect.  Contemporaries such as Boole and Robertson were very critical towards Jevons's use of mathematical symbols in logic.