Lord Halsbury L.C said at page 179,

An extensive power is confided to the justices in their capacity as justices to be exercised judicially; and “discretion” means when it is said that something is to be done within the discretion of the authorities that that something is to be done according to the rules of reason and justice, not according to private opinion…; according to law, and not humour. It is to be, not arbitrary, vague, and fanciful, but legal and regular. And it must be exercised within the limit, to which an honest man competent to the discharge of his office ought to confine himself…. So in Reg. v. Boteler, where justices thought proper not to enforce the law because they considered that the Act in question was unjust in principle, the Court of Queen’s Bench compelled them by a peremptory order to do the act which nevertheless the statute had said was in their discretion to do or leave undone. So, again, in the case of overseers who were required by 3 & 4 Vict. c. 61, to certify whether applicants for beer licenses were real residents and ratepayers of the parish, it was held that they were not entitled to refuse the certificate on the ground that in their opinion there were already too many public-houses, or that the beer shop was not required. So a discretion which empowered justices to grant licenses to innkeepers as in the exercise of their discretion they deemed proper would not be exercised by coming to a general resolution to refuse a license to everybody who would not consent to take out an excise license for the sale of spirits: Reg. v. Sylvester.