In 1966, Y. Imai and K. Is´eki [2] introduced a new notion, called a BCK-algebra. This
notion is originated from two different ways: One of them is based on set theory; another
is from classical and non-classical propositional calculi. As is well known, there is a close
relationship between the notions of the set difference in set theory and the implication
functor in logical systems. Then the following problems arise from this relationship. What
is the most essential and fundamental common properties? Can we formulate a new general
algebra from this viewpoint? How can we find an axiom system to establish a good theory
of general algebras? To give an answer these problems, Y. Imai and K. Is´eki introduced
a notion of a new class of general algebras which is called a BCK-algebra. This name is
taken from the BCK-system of C. A. Meredith. Since then many researchers studied several
notions and properties of BCK-algebras. For the general development of BCK-algebras,
the ideal theory plays an important role.