The major challenge of legal and political theory is to develop an account of what it means for people to be free. Historical efforts to answer that question typically have emphasized the correlative rights and duties for all individuals: No one thinks that any coherent notion of liberty lets everyone do just what they want, the rest of the world be damned. A world in which everyone can kill everyone else would breed massive individual and collective insecurity, and private contracts are not sufficient to either secure bodily integrity or protect property rights. There are impossible transactional obstacles in asking every person, born or unborn, to independently make arrangements with all others on these critical topics. Even in a free society, these basic rules of the game have to be defined and imposed collectively.

But there are limits. To echo the famous words of John Stuart Mill in “On Liberty,” it won’t do to say baldly that no individual is allowed to do any act that harms another. By that measure, market competition would need to end, because it harms losing competitors, and building on one’s own plot of land must be prohibited if it obstructs someone else’s view. Subjecting such actions to legal sanctions would leave no one with the freedom to do anything. Thus every legal system defines actionable harms relatively narrowly, usually restricting them to the use of force and fraud against other individuals. The duties imposed by such a regime are clear; they can be applied to any society large or small; and they place all individuals behind a perimeter of rights that will then allow them to develop their talents and personal relationships more or less as they please.

A reader should expect that any work called “On Freedom” would address the strengths and weaknesses of this standard account. But Cass Sunstein, a distinguished and prolific Harvard Law School professor, pursues a different, and ultimately unpersuasive, agenda his new publication. “On Freedom,” which is more of a pamphlet than a book, focuses largely on the psychological states of people grappling with illness, smoking, drinking, drugs and economic insecurity. His opening passage asks a rhetorical question: “Does freedom of choice promote human well-being? Many people think so.” But he sees a huge catch: “What if people do not know how to find their way?” In the modern world, he suggests, individuals lose their way when confronted with too many choices, about everything from where to live to whom to marry.

At this point he introduces his key notion: “navigability”—the best way to get from here to there. “When life is hard to navigate,” Mr. Sunstein writes, “people are less free.” Indeed, for Mr. Sunstein, these obstacles “create a kind of bondage.” Bondage, however, without a taskmaster. By that one verbal ploy, the author turns to talking about all-too-human mistakes that have nothing to do with political freedom.

A system of laws keyed to force and fraud maintains a relatively narrow scope. But preserving freedom becomes much more fraught if social and legal forces mean that someone—the reader is never quite clear who—has the right to step in to correct the full range of individual blunders. For Mr. Sunstein, two arcane tools help achieve his ambitious ends: “nudges” and “choice architecture.” Neither works.