The keystone of the rule of law is the idea of the government of laws rather than the government of men. The keystone of the government of laws is legal control over human discretion. The existence of widespread discretion is therefore directly inimical to the existence of a liberal order. Discretions need to be exercised on the basis of justice or some real justification or even of mere reason. An unfettered discretion is an opportunity for temptation and for arbitrary, insolent, discriminatory, intrusive, socially engineering and corrupt, government. Where there are fixed laws there is (more or less) certainty, there is certainly impartiality (equality before the law) and consistency. A person may stand upon his legal rights without fear or favour. Discretion, on the other hand, undermines justice.
Discretion may exist in the context of executive, judicial and legislative branches of the modern state.
Executive discretion is the most dangerous of all forms of discretion. This is because its impact upon the citizen is immediate and uncertain. Legislative discretion is uncertain but not immediate. Judicial discretion is immediate but not uncertain. Executive discretion, in suffering from the effect of immediacy additional to uncertainty, is open to the greatest possible abuse. The administrator has immediate unfettered power over the individual who stands at his mercy. The opportunities for arbitrary, insolent, discriminatory, intrusive and corrupt activity as well as totalitarian social engineering are maximised at this point.