Especially since Kant's day the concept of the "a priori" has generated controversy among philosophers. In a wider and less rigorous sense, one which seems acceptable to all disputants, "a priori" designates that knowledge or class of propositions which not only can be known to be true, but also are such that no experience can possibly disprove them or provide a counter-instance. In this sense the "a priori" is equated with the "non-empirical." Due to Kant's influence the "a priori" also is frequently taken to designate what is not only "non-empirical" but also "necessary."