An idempotent element in an algebraic system is one which obeys the idempotent law, that is, it satisfies the equation Under most circumstances it is usual to write this as
If the algebraic system in question falls under the additional laws necessary to carry out the required transformations then is convertible to and this in turn to
If the algebraic system satisfies the requirements of a boolean algebra then the equation amounts to saying is identically false, in effect, a statement of the classical principle of non‑contradiction.
We have already seen how Boole found rationales for the commutative law and the idempotent law by contemplating the properties of selective operations.
It is time to bring these threads together, which we can do by considering the so-called idempotent representation of sets. This will give us one of the best ways to understand the significance Boole attaches to selective operations. It will also link up with the statements Peirce makes regarding his dimension-raising comma operation.