"The sophistry,'" he says, "supplies a legitimate presumption against those who use it. Only for lack of sound arguments does one resort to it. [That is based on the implicit assumption that logically sound arguments are more convincing than fallacies. Experience is far from showing that.] As regards measures that are in themselves sound it is useless, or at least it cannot be indispensable. [The same implicit assumption, and experience, again, in no sense concords with it.] The sophistry presupposes he is in those who use it, or in those who adopt it, either lack of sincerity or lack of intelligence." Bentham's assumption is that the person who uses a fallacy recognizes it as such (insincerity) or that, if he fails so to recognize it, wanting in intelligenceAs a matter of fact many fallacies that are current in a given society are repeated in all sincerity by people who are exceedingly intelligent and are merely voicing in that way sentiments which they consider beneficial to society. Also implicit in Bentham's sermon is the assumption that lack of sincerity and lack of intelligence are uniformly harmful to society; whereas there are plenty of cases to go no farther than diplomacy where too much sincerity may be harmful, and other cases where a highly intelligent man may go wrong and do incalculable harm to a society by forcing certain logical policies upon it; whereas a stupid individual instinctively following beaten paths that have been counselled by long experience may be a blessing to his country.
Pareto is deprecating the assumed persuasiveness of the logical. On the contrary people are more influenced by sentiments and select the enthymeme of their choice to serve as justification for their beliefs and actions.(Vol.3: Mind and Society)