I encourage anyone, including the other discussants on this topic, who has not yet read A Brief History of Liberty to do so. It’s an admirable book and full of interesting insights. (Like all the books I find interesting, I also was not persuaded by all of it, but each chapter was stimulating and — dare I admit it? — fun to read.)
A lot of issues are on the table in this discussion, so let me start by making a bold assertion: Philip Pettit’s attempted distinction between “liberalism” and “republicanism,” based on his distinction between liberal “non-interference” and republican “non-domination,” simply ignores, or perhaps tries to define away, the tradition, or even the possibility, of “liberal republicanism,”  in which freedom from subjection to the arbitrary will of others is identified with liberty. Of equal importance, his formulation of freedom seems to me merely a self-contradictory variant of the “positive liberty” limned by Berlin.
Pettit dismisses mere “non-interference” as a criterion of liberty on the grounds that one could be subject to, say, a monstrous tyrant who merely forbore from interfering (or in some way failed to interfere or refrained from interfering), and in such a circumstance one would not then be free, for one would be dominated by that tyrant’s arbitrary will. Liberal republicans agree. Pettit asserts that one could be subject to a power that interfered, but without being dominated, if the interference is not arbitrary, but furthers my interests:
As I may suffer domination without interference, so I may undergo interference without being dominated: without relating to anyone in the fashion of slave or subject. Suppose that another person or agency is allowed to interfere with me but only on condition that the interference promises to further my interests, and promises to do so according to opinions of a kind I share. 
One might ask “allowed by whom,” but Pettit assures us that we needn’t worry about that, for the interferee’s consent is irrelevant to the matter; all that matters is “the permanent possibility of effectively contesting it” and that the state’s “interference has to be guided by certain relevant interests and ideas and those interests and ideas are shared by those affected.”  (Of course, once the interferee has been interfered with by being arrested and imprisoned, he’s not likely to be able to effectively contest the interference and, in any case, even pre-arrest, he has to have secure rights which the guardians of his virtue — or “interests” or “ideas” — cannot override, regardless of whether Professor Pettit thinks it’s in the interest of the interferee to suffer such interference. Pettit’s theory undermines itself.)
Moreover, the fact that someone has to interfere with an agent, presumably by use of coercive force (and be in no doubt that that is what Pettit has in mind), suggests that at some level the agent does not share the interferer’s understanding of her interests, or that their opinions are not in fact shared. Otherwise, interference would not be necessary. I read Pettit’s book with some care and I fail to see how his theory is anything other than an example of Berlin’s “positive liberty,” for despite the agent’s ardent profession of her desire, Pettit’s “non-mastering interferer” is acting to advance (or his actions “promise” to advance) her real interests, or her real opinions, or the interests or opinions of some occult true self, that is, what the agent really needs or wants. We end up back in sixteenth century Geneva, but with the role of John Calvin played by Nancy Pelosi. No thanks.
Pettit asserts in his contribution to this debate that his view “is not equivalent to positive freedom, since it requires the absence of domination,” but “positive freedom” (in Berlin’s use of the term, not the sense of “wealth” also deployed by Schmidtz and Brennan) also requires that the interferer not dominate, in Pettit’s sense of being arbitrary, for the interference of Plato’s guardian class is guided by your real interests, by what you really want, but are too ignorant or deluded to know that you want it, or too weak to act to secure it. Berlinian positive freedom also requires absence of domination, as defined by Pettit, for the “non-mastering interferer” in such cases is merely “forcing you to be free” by forcing you to do what you really want to do. As Pettit puts it in his contribution to this debate, “you may be interfered with without being subject to the interferer’s will, as when that agent – like Ulysses’s sailors – interferes on your terms: that is, by your own will.” Note, however, that Ulysses consented to being tied to the pole, and asked the sailors to put wax in their ears. Pettit and his Genevan republicans aren’t interested in that kind of very robust consent when it comes to determining “your own will.” They have their own ideas of what’s good for you, and you’re going to get it good and hard. 
Pettit takes issue with Schmidtz and Brennan on the role of normative judgment in providing guidance to empirical research on the feasible and I think he’s right on that point. Where I find fault with his approach is in his failure to consider “liberal republicanism” as an option and his disguising of the “positive” (i.e., real, authentic, noumenal, etc., etc.) freedom as somehow a third kind of freedom. It doesn’t work.
John Christman in his contribution to this discussion strikes a concordant note with Pettit and me when he notes, in response to the call by Schmidtz and Brennan to get out of our armchairs, that “we must be able to specify what ‘liberty’ means and when such policies have these aims.”
Interestingly, we find him subjecting not only the state to such questions of efficacy, but also all institutions, including business corporations, and, one might assume, families, labor unions, churches, synagogues, mosques, temples, chess clubs and soccer leagues, but it seems clear that in all cases it is the state that will decide whether the soccer leagues and companies and mosques measure up, and not the other way around.  The truly odd passage was his puzzlement about whether Google or the Chinese state is responsible for censorship:
For example, when the Chinese government puts pressure on Google to censor its content and the company considers shutting down its Chinese operation, who should we say is responsible for robbing Chinese citizens of access to the site? That question is complicated of course, but even if one places the blame on the doorstep of the government of China, the fact that Google is the only major search engine that can deliver what is needed by the Chinese users is also clearly relevant to the appraisal of the extent of their liberty.
Professor Christman could have roused himself from his armchair for a minute to access, say, Yahoo.com (which can’t be that far from his armchair), where he could have typed in (as I just did) “Google China market share” and found that Google had less than 30% of “the market” in China, with state-supported Baidu coming in at about 60%. But after Christman settles back in his armchair, he should wonder why people such as I find it truly bizarre, and even odious, that he thinks that an organization that makes knowledge available, merely by existing, might be as responsible as the Chinese government censors, armed with the power to arrest and kill, for “the limitation of the freedom of Chinese dissidents.” I could give similar examples of blaming the victims for their victimization, but I would rather leave them to the thoughtfulness of the readers to imagine all the ways in which the existence of targets for rights-violations are as responsible for their loss of freedom as the criminals who assault them.
Finally, and consistently with the thesis of his book The Myth of Property: Toward an Egalitarian Theory of Ownership, he blames those who create wealth for the lack of wealth. In that book, he argues for taxing away all profits and (without leaving his armchair) concludes that it would have no negative economic impact, for “liberal ownership is neither necessary nor sufficient for competitive markets.”  (“Liberal ownership” includes the various rights to control what is owned, and – Christman’s target – the right to “gain income” from it. ) Entrepreneurial profits, or economic residuals, “occur only in imperfect markets and hence result from artificial scarcities.”  You see, under conditions of “perfect competition” (and Christman clearly is misled by the term “perfect” to consider it normative, and not merely an unfortunate term for an abstract model designed to isolate features of economic interaction), all residuals would be zero, so….(cue the drum roll, please…) when you tax them all away, you are merely approximating perfectly competitive markets, and what could be more perfect than that?  In Christman’s world, allowing the state to confiscate 100% of “economic rent” (i.e., income above opportunity costs, or what a factor of production could earn in its next best productive use, or the excess above what would be necessary to induce its owner to deploy it in that way) or entrepreneurial profits, would have no negative impact on economic performance. The positive effect of market exchange “is not diluted if all of the surplus goods due to artificial scarcities that produce rent were confiscated, say, by the state.”  Very bracing stuff. And no doubt the result of many trips out of Professor Christman’s armchair to examine regimes where economic rents are all seized by the state.
That digression is necessary to give some context to Christman’s concluding remarks about the world.
Finally, for many of us, the track record of economic forces and the workings of competitive markets in effectively improving the lives of citizens in an equitable and morally acceptable manner, in a world where over one billion people still live on less than one dollar a day with no foreseeable prospect of meaningful improvement from market innovations, is already an obvious and dismal failure. I applaud the call for all of us to leave our armchairs and evaluate the effects of actual policies, but for those of us who have done so and looked at a woefully under-served global population, we can say “thanks but we’ve seen quite enough.”
Evidently what Christman sees are snapshots, and not motion pictures. He should go to the movies sometime. If he went to the world economic development movies, he’d see rising incomes coinciding with better-defined property rights and freer trade. He’d see the incomes of the poor rising and not only the percentage of world population living under severe poverty declining, but the absolute number, as well, even in the face of population increase. He could even use the wonder of the internet to access from his armchair the detailed statistical studies of Angus Maddison, Deirdre McCloskey, David Dollar, and many others, who document the astonishing rise in per capita incomes around the world. He could even see moving charts that the amazing Angus Maddison has created to illustrate the changes. And he could wonder whether economic processes of trying to create or find, and then claim, economic residuals (i.e., the entrepreneurial profits that Christman would confiscate) under conditions of the 3-Ds (property rights that are Definable, Defendable, Divestible) might account for the declining percentage and number of people living in abject poverty.
Finally, to turn to Schmidtz and Brennan’s defense of their approach to liberty, I think that they are on to something important when they argue, contra G. A. Cohen, that the net amount of freedom is not a constant, but does change from setting to setting. There is certainly more freedom when all can drive according to known rules, so that we do not crash into each other willy nilly. Traffic laws and property laws both allow us “to stay out of each other’s way,” and property laws also enable us to truck, barter, and exchange one thing for another. That raises the question of whether law is always “coercive,” which seems to be a theme of both Pettit and Christman. Clearly not. If we understand law in Lon Fuller’s sophisticated understanding as “the enterprise of subjecting human conduct to the governance of rules”  law can be coordinative without being commanding or coercive. And better laws, with less reliance on arbitrary power, can enhance liberty. Much of Schmidtz’s and Brennan’s fine book is on precisely the history of the rule of law.
I robustly disagree, nonetheless, with Jason Brennan’s defense of what I would consider a promiscuous use of the term “liberty.” Not only does it lead to the issues of “tradeoffs” of liberty (e.g., “more education” vs. less freedom to control one’s income, or to move, but note that the latter must be called “freedom,” and the former can easily be called “education”) that Pettit, Christman, and even Schmidtz and Brennan acknowledge (but in the latter case wish to avoid), but I think it drains clarity from the discussion. If I have understood him correctly, Brennan wants to listen to how people use a word (e.g., the adjective “free” or the adverb “freely”) and then call all of the cases to which they refer, in which one can be free or act freely, cases of “freedom.” So if I tip freely or drink freely, I am free, and if a benefit is free (“free of charge”), then it is a case of freedom, etc., etc.
I will concede freely, as it were, that it is meaningful and not confused to say that I can be freed from being pinned under a tree trunk and that I can be said to be “cancer free,” but when using the term in discussions of political philosophy, we should speak clearly, meaning that we should exclude certain uses of terms. A field of corn may be “free of pests,” but it would be odd to call the field or the corn “free” or, more importantly, to say that it enjoys freedom. Similarly, the child born in the USSR in 1970 invoked by Brennan is free of polio, and thus a healthier person, but not a freer person, than a child born in Britain in 1935, twenty years before the polio vaccine was announced. When discussing human history, rather than natural history (in which we might say that a tectonic plate was freed from another and shifted) we ought to talk about human behavior, meaning about those restraints on humans for which other humans can be held accountable. We need language that allows us in a discussion of the history of freedom to distinguish between trees falling on us and lynch mobs falling on us, or our discussion will be incoherent, just as a discussion of “games” will be more focused and coherent if we distinguish what game we are discussing and even what aspect of that game (strategic interaction, fun, rough-housing, etc.).
Let’s take another important term — happiness — and see whether Brennan’s approach helps us to clarify our meanings. I sometimes hear people say things such as “ice cream makes me happy,” “I am happy to see you,” “I don’t feel happy,” “she finally knew happiness,” and “happiness is an activity in accordance with human virtue.” It refers now to a state of gratification, later to a state of joy, yet later to a state of contentment, and finally to a life well lived. I sometimes shock younger people out of their equivocal uses of terms by pointing out that “sadness is a part of a happy life,” and that a sad person can be a happy person, if that person is sad for the right reasons and on the right occasion, etc., etc. When parents die, most normal children are sad, and being sad on those occasions is a part of a happy life. How horrible if they were to be as perky on the day of their parents’ funerals as when their parents were hale and hearty and were to fail to mourn properly. How odd it would be if we were to count all of the states above as “happiness” (in contrast to the happiness of a life well lived) and were to count the ever-perky person who is oblivious to occasions for sadness as “happier” than the person who has endured sadness for the right reasons and incorporated that sadness into a happy life. It’s important to distinguish happiness, in the sense of a life well lived, from gratification, joy, contentment, and other states of being, even if we sometimes use the term “happiness” promiscuously to refer to all of them. Similarly with terms such as justice, goodness, and the like.
When we define freedom as the attainment of all the good things we may want (for when we get them, we are free from their lack) and the avoidance of all the bad things we may wish to avoid (for when we avoid them, we are free of them), we drain the word freedom of meaning. If “freedom” simply means “goodness,” it no longer serves any useful role in discourse.
Just because people often use terms without regard to relevant distinctions is no justification for failing to make those distinctions when we wish to think clearly about hard problems. It’s an especially serious problem when one wants to ask whether one society is freer than another, or whether the society is freer earlier or later. It seems to me that Brennan concedes my point with regard to the thought experiment I posed. He quite reasonably admits that “Nazi Germany was not a free country.” Why wasn’t it? They did, after all, have more “freedoms” (Autobahns, vaccines, etc.) in his “positive” sense (not to be confused with Berlin’s sense) of the term. He admits that the regime “truly was evil and oppressive” (although one might ask what “oppressive” means, other than denying freedom). But to point out that it was evil is evading the issue. Brennan concedes that it was “not a free country,” despite there being more of one of his categories of “freedom.” Why don’t they balance out? Brennan wants to reserve only one (“negative”) sense of the term to describe the country, but “the country” is an aggregate of people and their relations and he has conceded that the people and their relations are, with regard to Autobahns and vaccines, in his terms, more “free.” So why isn’t Nazi Germany a “free country,” freer than the Germany of fifty years earlier? How many Autobahns (more freedom) might balance out how many arbitrary arrests (less freedom)? He could have avoided the problem by simply admitting that the term freedom should be reserved (“monistically”) for social relations (as he implicitly does when he affirms that Germany in 1939 was “not a free country”) and using terms such as “wealth,” “ability,” and health for telephones, Autobahns, and vaccines. I am sure that no one would understand him any less well, and at least some would understand him better. That would clearly be a Pareto improvement, even if it wouldn’t make anyone freer.
 See the complex interaction of classical republican ideas and modern liberal ideas in the work of Joyce Appleby, e.g., Liberalism and Republicanism in the Historical Imagination (Cambridge, MA: Harvard University Press, 1992), which is nowhere cited in Philip Pettit’s own Republicanism: A Theory of Freedom and Government (Oxford: Oxford University Press, 1997).
 Pettit, Republicanism, p. 23.
 Pettit, Republicanism, pp. 62-63.
 Thomas Hill Green sought to “free” his fellow Englishmen by prohibiting them the enjoyment of alcohol: “The citizens of England now make its law. We ask them by law to put a restraint on themselves in the matter of strong drink. We ask them further to limit, or even altogether to give up, the not very precious liberty of buying and selling alcohol, in order that they may become more free to exercise the faculties and improve the talents which God has given them.” ‘‘Lecture on Liberal Legislation and Freedom of Contract,’’ in Works of Thomas Hill Green, ed. R. L. Nettleship (London: Longmans, Green, and Co., 1906), p. 386.
 That is a point he makes abundantly clear in his book The Myth of Property: Toward an Egalitarian Theory of Ownership (Oxford: Oxford University Press, 1994), p. 26: “The exact structure of control and income rights will be determined by the distributive principles of the state and hence justified according to normative argument.” “State” in this sense may mean the social order, but Christman also focuses our attention on the apparatus of the state as such, which is privileged above all other forms of human interaction, as he makes even clearer later: “the state is authoritative no matter what property rights structure it enforces….” (p. 34)
 Christman, The Myth of Property, p. 28.
 Christman, The Myth of Property, p. 29.
 Christman, The Myth of Property, p. 131.
 It would have helped Christman to have read Harold Demsetz’s monograph Economic, Legal and Political Dimensions of Competition (Amsterdam: Elsevier Scientific Books, 1982), which clarifies the model and eliminates any confusion about how “perfect” such conditions are.
 Christman, The Myth of Property, p. 170.
 Lon Fuller, The Morality of Law (second revised edition; New Haven: Yale University Press, 1969), p. 122.