For most unlawful acts, we can all pretty much agree about the egregiousness of the offense: Murder is worse than burglary is worse than parking violations. But when it comes to illegal immigration, that consensus breaks down dramatically. As undocumented immigrants and their supporters see it, these residents are doing what any of us would do — seeking better lives, often driven by intolerable poverty. To opponents, illegal immigration is not only a crime, but a crime that takes jobs from American citizens. From this point of view, the punishment — simply being sent home — can even be seen as lenient.The recent Arizona legislation to step up enforcement of immigration law embodies the second outlook — and has returned the issue to the fore of national debate, triggering protests by those who believe we should ease up on illegal immigrants rather than crack down on them.

Why should we decide not to enforce a law? In an upcoming book, the political scientist Joseph Carens stakes out a moral case for letting immigrants, even illegal ones, stay. He accepts the prerogative of states to deport unauthorized immigrants — up to a point. But after the newcomers have established lives and ties here, he argues, they have acquired what he calls the “right to stay,” a moral claim to a place in the society where they’ve settled. When enough time has elapsed, he suggests, deportation no longer amounts to sending people home but rather uprooting them from home. Instead, Carens submits that after a designated period — he proposes five to seven years — undocumented immigrants should be embraced as Americans and granted the chance to formalize their status.

Raised mostly in Wellesley, Carens himself emigrated to Canada, where he is a professor at the University of Toronto and a dual citizen. The book, “Immigrants and the Right to Stay,” to be published in September by MIT Press, includes his essay and responses from six other experts. He recently spoke with Ideas by telephone from his home in Toronto.

IDEAS: You write that after a certain period of time, people who entered the country illegally acquire a “right to stay.” On what grounds do they have this right?

CARENS: The fundamental argument is, over time people become members. Regardless of the terms under which they entered, the situation, the conditions under which they entered, they become members.

IDEAS: What does the word “member” mean to you?