The purpose of discourse is derived from the powers of the mind to which they appeal (understanding, imagination, passions, will), rather than the classical three, which are based on the public purpose of oration. The classical categories (see Cicero and Quintilian) are the demonstrative, to praise or blame; the deliberative, to advise or dissuade; and the forensic, to accuse or defend. In considering each of these, Campbell believes that not only understanding and memory of the audience must be taken into account, but the orator must as well provide particular attention at stimulating their passions. To incorporate this was as an obvious concern for Campbell, who believed that effective preaching must be measured by its effects on the audience.