Volunteers help around 150 refugees and migrants, mostly from Syria and Iraq, to disembark from a vessel after their arrival from the Turkish coast to the northeastern Greek island of Lesbos, on Tuesday, Dec. 1, 2015. (AP Photo/Santi Palacios)

It is no longer news that millions of Syrians have fled their country, and that they are streaming in droves through the Middle East and into Europe. They are seeking haven both from a civil war, in which a host of opposition forces are attempting to unseat Bashar al-Assad’s government from power, and from the violence of the so-called Islamic State, a militant jihadist group which says it is attempting to install a “true” Islamic caliphate in Syria and Iraq. Many, including France, now refer to the group as Daesh, an acronym that does not recognize the group’s claim to statehood. Daesh’s attempt to create a caliphate involves mass killings and enslavement of thousands of people it declares to be infidels.

Daesh is also claiming responsibility for multiple recent terrorist attacks outside of the territory it is seeking to control, most notably, for the recent attacks in Beirut and Paris, which left nearly 200 people dead between them.

Early newspaper reports suggested that one of the Paris attackers entered Europe by claiming to be a refugee. That’s unproven, and very likely false. Nevertheless, voices across democratic countries — including across Europe, as well as in Australia and the United States — are calling on governments to reconsider earlier commitments to offer haven to refugees. Many more, including the United States, are demanding more robust security screening for refugees.

On the one hand, states are obligated, by international law and basic morality, to admit refugees in need of haven. On the other hand, states are also obligated to protect the security of their citizens. How should these obligations be balanced with one another? A sober assessment of the claims on both sides offers no easy answer.

The continuum of arguments on behalf of open borders and closed borders

For some political theorists, so-called “open border” theorists, the right of states to control their borders is limited, for two main reasons. One is individual freedom: where individuals cannot freely cross borders in pursuit of their objectives, their freedom is objectionably limited. The other focuses on the extreme material inequalities between states, and suggests that protecting borders entrenches and protects these inequalities. In both these lines of thinking, giving individuals the ability to cross borders is one way to erode them. Therefore, borders ought to be as porous as possible, and countervailing reasons against the freest of possible movement are greeted by open border thinkers with skepticism.

For other political theorists, sometimes lumped together under the label “closed border” theorists, states do have the right to exclude. They possess this right because states offer valuable goods to their members, which may sometimes legitimately trump the “extra” freedom involved in crossing borders. These goods include a shared cultural environment and a closed community in which to practice the redistribution of wealth and democratic politics, all of which depend on extensive stores of trust. Trust is more easily extended where values, norms and interests are believed to be shared. If too many newcomers — who may not share these values, norms and interests — arrive too quickly, it can threaten the stores of trust on which these goods depend.

We can therefore understand closed and open borders theorists on a kind of continuum. Those on one side of the continuum prioritize the valuable goods that a state provides. Those on the other prioritize the freedom of individuals to move.

What this means for migrants and refugees from Syria

How does this apply to the refugees from Syria? Political theorists rank migrants in terms of how valuable it is for them to be able to cross borders, or examine which of their rights will be protected or enhanced by crossing borders. Potential migrants may have all kinds of reasons to cross borders: to escape persecution, to be with family, to leave behind endemic poverty, to be with others of their own religion, to study, to pursue a career in opera, to seek adventure, and so on.

All political theorists agree that refugees — those seeking refuge from persecution on the basis of characteristics they cannot change — have the greatest need to move. But only some believe this obligation always trumps the right of states to exclude. For the latter thinkers, state sovereignty and the goods it provides are valuable enough to trump refugees’ claims, and in fact to trump all migrants claims for admission. Most others, however, believe in assessing and ranking the reasons migrants request entry, and evaluating their claims against the positives and negatives that could result if they enter. This ranking acknowledges that, along all migrants, refugees have the most credible and urgent claim.

But what happens when it is alleged that refugees pose a security risk? Not simply a risk to democratic institutions or a cultural community or to a group’s willingness to significantly redistribute wealth from those who are most well-off to those who are least well-off, all of which has been claimed of refugees and immigrants in the past. What if there’s a danger to the physical security of citizens and their homes?

The claim that Syrian refugees pose this risk must be taken seriously. That claim threatens to undermine a global asylum system that, while imperfect, has gotten many vulnerable individuals to safety. Borders must remain open to refugees for this system to keep operating.

The threshold of proof for closing borders against refugees in the name of security

Even open borders advocates concede that public order can be a legitimate reason to tighten border control, although they urge skepticism about claims that public order is indeed at risk. If there is a genuine fear for the public order of the welcoming state, even they agree that migrants can be excluded. National security is among the strongest reasons to close borders. But open border thinkers say we must be certain that such concerns are genuine; the threshold of proof is very high.

This skeptical attitude acknowledges the profound importance of security, and also acknowledges that a state’s legitimacy is in part measured by its capacity to provide security. But this attitude then demands that we know a great deal before we can justifiably exclude migrants in general and Syrian refugees in particular. We must know, first, whether Syrian refugees are a risk to security; second, whether the risk is substantial enough to justify excluding them; and third and finally, whether there are alternative ways to mitigate this risk that does not demand exclusion.

Political demagogues are using the Paris attacks to justify anti-immigrant agendas. Nevertheless, policymakers must soberly examine the evidence available before taking such a harsh stand.

Patti Tamara Lenard is associate professor at the Graduate School of Public and International Affairs, University of Ottawa, Canada.  Her 2012 book is titled “Trust, Democracy and Multicultural Challenges.”