A “presumption of X” signifies that X is taken to be the case without requiring argument or evidence to support it. The burden of its rebuttal is on the challenger of the presumption.
The presumption of liberty signifies that an individual is taken to be free to perform any feasible act without having to show that there is no sufficient reason that he should not perform it. The burden of showing sufficient cause against the act (or against this particular individual performing it) is on the challenger. The latter is characteristically the political authority, or the plaintiff in juridical disputes, but may be anybody with access to public rulemaking and administration. Sufficient cause may consist of an applicable rule prohibiting the act or in enough probability that the act, if committed, would significantly harm another person or persons.
Conventions, customs, and laws make up the rule system intended to guide social behavior. A rule system that runs in terms of prohibitions is favorable to the presumption of freedom: “everything that is not expressly prohibited is (liable to be) free.” A rule system that runs in terms of permissions, such as bills of rights, favors the presumption of unfreedom: “everything that is not expressly authorized is (liable to be) prohibited.” However, although the nature of the rule system may favor one presumption or the other, it does not logically establish or exclude either.
It is both widely held and widely contested that the presumption of liberty is the most solid foundation one can find for the edifice of classical liberal or libertarian theses.
One sharp criticism of the foundational role of the presumption of liberty, which dismisses it as “an implausible doctrine,” condemns it mainly on the ground that it is undiscriminating and gives the same weight to vitally important as to trivially unimportant freedoms. Condemning it on this ground is like condemning the umbrella on the ground that it protects indiscriminately from the rain not only the great and the good, but the ordinary plodders as well. There may be a case for ranking liberties in a hierarchical order of greater and lesser, as was done notably by John Rawls and his followers, but it is not a liberal case if only because it must invest someone with the power of deciding which liberty is greater than another.
The other, more weighty, attack against the presumption of liberty holds that it depends on the love of liberty or, more precisely, on the imputation of a value to liberty. This dependence makes the presumption of liberty doubly vulnerable. It may be argued that liberty as such has no value and that those who claim a preference for it really like not liberty, but other values they think it would bring, but that can be obtained more directly and more safely without recourse to the presumption of freedom. It also can be argued, reasonably enough, that, although freedom may be valuable, it is not the sole value we pursue. Many believe, rightly or wrongly, that other values in effect compete with freedom, with more freedom causing less equality or security. Hence, the presumption of freedom, far from being a firm foundation, hinges precariously on the tradeoffs that happen to prevail among these competing values.
This line of argument is a mistake. The presumption of freedom in no way depends on the love of freedom. It is a pure product of logic and epistemology. Suppose opinion is divided over whether a certain act is (or should be treated as) free. The actor who wishes to perform it contends that it is free, whereas challengers contend that it is not. Propositions that have descriptive meaning are either verifiable or falsifiable or sometimes both. Take the proposition that the act is not free. There may be indefinitely many potential reasons for believing this notion, having mostly to do with the meanings of rules and with putative harms to various interests. The would‐be actor can falsify some of them by showing that they are groundless or of insufficient force. But as he falsifies some, others can always be advanced because such reasons are numberless, and although the actor may falsify any large number, he cannot falsify all. Therefore, the actor cannot bear the burden of proof, and, in logic, it would be nonsensical to place it on him.
The challenger, however, is in an altogether different, we may say asymmetrical, position. Any specific reason against the act in question that he thinks is sufficient can be verified by argument and evidence bearing on that reason. If he fails to verify his first reason, he can find a second and marshal evidence supporting that until he either finally succeeds in his challenge or runs out of verifiable arguments. There is nothing logically nonsensical or epistemologically unfeasible about the task of verifying his proposition that the act should not be free. Consequently, he can carry the burden of proof. Until he succeeds in discharging it, the presumption of liberty prevails, as it were, by default.
The presumptions of innocence and property are twin sisters of the presumption of liberty, in that they spring from the same asymmetry between verification and falsification. “You have committed a crime” and “your title to this property is invalid” are unfalsifiable, but verifiable, statements open to proof, but not to disproof. The presumption of innocence does not depend on the “rights” of the accused nor the presumption of property on the economic case for the owner’s security of tenure. Like liberty, they owe their status to the nature of the means the mind possesses for telling true from false.