The essence of political liberalism is the belief that the burden of proof ought to be placed on those who would oppose negative liberty or advocate prohibition in cases where no direct harm is being perpetrated on someone by the act which one argues ought to be prohibited. We have the right to negative liberty, that is, the freedom to do whatever we want, and this freedom is axiomatic. The burden of proof is on those who would oppose this, and they must demonstrate why or how a particular act impinges upon the liberty of others in some way, in order for the act to be opposed(Gaus & Courtland, 2011).
Basic to this is the social contract theoretical notion that man exists in a 'state of nature' in which he is free, and we must justify political authority in light of this. Some, like Rousseau and Hobbes, come to what we might normally think of as 'illiberal' conclusions, but strictly speaking, they are liberals insofar as they hold to this social contract theory model of the human person. In the case of someone like Hobbes, however, he legitimates authority insofar as man requires significant prohibition, according to him, to protect others from his behavior (and to protect him from the behavior of others) (Gaus & Courtland, 2011). "Insofar as they take as their starting point a state of nature in which humans are free and equal, and so argue that any limitation of this freedom and equality stands in need of justification (i.e., by the social contract), the contractual tradition expresses the Fundamental Liberal Principle"(Gaus & Courtland, 2011). The concept of negative liberty of which liberals tend most commonly to be advocates was perhaps most famously articulated in contemporary political philosophy by Isaiah Berlin.
I am normally said to be free to the degree to which no man or body of men interferes with my activity. Political liberty in this sense is simply the area within which a man can act unobstructed by others. If I am prevented by others from doing what I could otherwise do, I am to that degree unfree; and if this area is contracted by other men beyond a certain minimum, I can be described as being coerced, or, it may be, enslaved. Coercion is not, however, a term that covers every form of inability. If I say that I am unable to jump more than ten feet in the air, or cannot read because I am blind…it would be eccentric to say that I am to that degree enslaved or coerced. Coercion implies the deliberate interference of other human beings within the area in which I could otherwise act. You lack political liberty or freedom only if you are prevented from attaining a goal by other human beings (Berlin, 1969: 122).
For Berlin and those who follow him, then, the heart of liberty is the absence of coercion by others; consequently, the liberal state's commitment to protecting liberty is, essentially, the job of ensuring that citizens do not coerce each other without compelling justification. So understood, negative liberty is an opportunity-concept. Being free is merely a matter of what we can do, what options are open to us, regardless of whether or not we exercise such options (Taylor, 1979)(Gaus & Courtland, 2011).
This is opposed to positive liberty. The concept of positive liberty was first articulated by Rousseau, who conceived of freedom in terms of acting according to one's "true" will. The concept received further articulation by neo-Hegelians like Bosanquet and Green, who articulated the concept of liberty in terms of whether or not one was acting according to one's true nature. For Green, for example, one might be 'free' to use heroin if it is not against the law, but one is not truly free if one is using it because of addiction or (Gaus & Courtland, 2011).
For Green, a person is free only if she is self-directed or autonomous. Running throughout liberal political theory is an ideal of a free person as one whose actions are in some sense her own.In this sense, positive liberty is an exercise-concept. One is free merely to the degree that one has effectively determined oneself and the shape of one's life (Taylor, 1979). Such a person is not subject to compulsions, critically reflects on her ideals and so does not unreflectively follow custom, and does not ignore her long-term interests for short-term pleasures. This ideal of freedom as autonomy has its roots not only in Rousseau's and Kant's political theory, but also in John Stuart Mill's On Liberty. And today it is a dominant strain in liberalism, as witnessed by the work of S.I. Benn (1988), Gerald Dworkin (1988), and Joseph Raz (1986); see also the essays in Christman and Anderson (2005).
This Greenian, autonomy-based, conception of positive freedom is often run together with a very different notion of ‘positive’ freedom: freedom as effective power to act or to pursue one's ends. In the words of the British socialist R. H. Tawney, freedom thus understood is ‘the ability act’ (1931: 221; see also Gaus, 2000; ch. 5.) On this view of positive freedom, a person who is not prohibited from being a member of a Country Club but who is too poor to afford membership is not free to be a member: she does not have an effective power to act. Although the Greenian autonomy-based conception of positive freedom certainly had implications for the distribution of resources (education, for example, should be easily available so that all can develop their capacities), positive freedom qua effective power to act closely ties freedom to material resources. It was this conception of positive liberty that Hayek had in mind when he insisted that although ‘freedom and wealth are both good things…they still remain different’ (1960: 17-18)(Gaus & Courtland, 2011).
The Republican understanding of freedom, advocated by Machiavelli and Cicero, was to define freedom in terms of whether or not one lived in servitude of another or others (Gaus & Courtland, 2011). One was either free or dominated by another(Gaus & Courtland, 2011). The ideal liberty-protecting government, then, ensures that no agent, including itself, has arbitrary power over any citizen"(Gaus & Courtland, 2011). Once such freedom from domination has been secured, republican theorists tend to be unconcerned with questions of developing one's capacity to realize one's potential or act in accord with their nature, at least from a political perspective. In this sense, they oppose advocates of understanding liberty as 'positive' liberty(Gaus & Courtland, 2011). It is not clear, however, whether or not, or to what extent, the republican conception of liberty can be compellingly differentiated from the liberal conception of negative (Gaus & Courtland, 2011).
It ought to be kept in mind, however, that there are different kinds of liberals. Classical liberals emphasize the importance of having a political system consistent with the notion of private property. To be free, for the classical liberal, is to be free to own private property. This is one reason the term 'classical liberal' is oftentimes used interchangeably with 'libertarian.' Such a right to private property is seen by classical liberals not only as the expression of liberty par excellence, but as an essential element in securing other liberty.
Here the idea is that the dispersion of power that results from a free market economy based on private property protects the liberty of subjects against encroachments by the state. As F.A. Hayek argues, ‘There can be no freedom of press if the instruments of printing are under government control, no freedom of assembly if the needed rooms are so controlled, no freedom of movement if the means of transport are a government monopoly’ (1978: 149)(Gaus & Courtland, 2011).
There is a broad spectrum of beliefs associated with classical liberals. Some classical liberals see taxation, for example, as legitimate insofar as it secures protection of liberty. Others reject the notion of taxation as inherently illegitimate(Gaus & Courtland, 2011). "Most nineteenth century classical liberal economists endorsed a variety of state policies, encompassing not only the criminal law and enforcement of contracts, but the licensing of professionals, health, safety and fire regulations, banking regulations, commercial infrastructure (roads, harbors and canals) and often encouraged unionization"(Gaus & Courtland, 2011).
Gaus, Gerald and Courtland, Shane D., "Liberalism", The Stanford Encyclopedia of Philosophy (Spring 2011 Edition), Edward N. Zalta (ed.), URL = <plato.stanford.edu/archives/spr2011/entries/liberalism/>.