When we think of universities, we picture tree-lined campuses with stately buildings. But the campus is only one aspect of what a university is (not least in this age of distance education). Bricks and mortar give shape to a deeper identity -- that of a community of teachers and students with shared personal, professional and societal goals.

A large part of a university's identity is bound up with its desire to be open; indeed, schools constantly struggle to reconcile accessibility with security. How, and on what rationale, should any citizen be barred from the property, especially that of a public institution?

This question is particularly acute at some of the country's venerable urban universities. Consider, for example, the hard neighborhoods that border Columbia, Penn and the University of Chicago.

But we've also seen it become an issue in a more pastoral setting: Virginia Tech. It is idle to pretend that such a sprawling campus could be effectively fenced in.

But beyond the university as physical community is an even deeper identity: the university as idea.

The first European universities in the medieval period established a tradition of separation from what we in the ivory tower today label "the real world."

Quite apart from the close association of Christianity with higher education in the Middle Ages, there is some reason to regard the real world as the secular component of our geography. One still occasionally reads references to universities as "cathedrals of learning."

These tropes were on my mind one Monday evening as I thought about ancient Greek sanctuaries -- the subject of a classroom lecture I was preparing for the next day. Magnificent sanctuaries such as those at Athens, Olympia and Delphi provided Greek architects and sculptors with venues for their highest artistic achievements, all in service to the rituals of pagan religion.

I ended up pondering the more abstract ancient concept of sanctuary. The Greeks, as they say, had a word for it. By custom, anyone who sought the protection of a god by clinging to an altar was asylos, or inviolate. The word is related to the English term asylum.

During the Greco-Persian Wars of the early fifth century B.C.E., nothing was a clearer mark of Persian barbarity than their sacking of the temples and religious statuary on the Athenian Acropolis. After the Battle of Salamis in 480 B.C.E., the Athenians climbed the Acropolis and found at its altars the bodies of soldiers whom the Persians had slaughtered after successfully breaching the walls.

According to tradition, this horrific discovery stiffened the Greeks' resolve to turn back the enemy, which they did once and for all the following year. For some three decades, no reconstruction took place on the Acropolis, the result (according to two ancient authors) of a decision to leave the monuments in ruins forever as a monument to Persian impiety.

The ancient Greeks -- indeed, Europeans to this day -- have preserved a respect for the context of sanctuary, one that has, curiously enough, morphed into a secular context. In Europe, universities are regarded not merely as physical locations where ideas and free speech can flourish but as quasi-sacred institutions.

Again, Greece provides an instructive example.

The country was ruled by a fascist junta between 1967 and 1974. A leftist student demonstration against the government that arose in 1973 at the Polytechneion (the National Technical University) in Athens was brutally put down after three days, when army tanks rolled over the iron gates of the campus. In the chaos that ensued, a yet-uncertain number of students died.

The horror of the crackdown was not measured only in the loss of life and damage to buildings, though. The violation of the university resounded as a symbolic transgression, and it evoked outrage on the part of governments and people all over Europe. Invading universities is something that civilized people simply do not do.

In the United States the concept of sanctuary, which denies to government uninvited entry, has taken hold only in our religious institutions. Consider the congregations that have in recent years succored undocumented immigrants, mostly with impunity.

The concept has not manifested itself in the university, however. (Recall the fatal confrontation between Ohio National Guard troops and protesting students at Kent State after the Cambodian invasion in 1970).

The decades since the Vietnam War have seen a number of social transformations. The American university has evolved into an increasingly corporate entity. Legislators see the university as the factory whose mission is to manufacture a skilled work force, and many in the public regard higher education as primarily a meal ticket.

As the massacre at Virginia Tech has demonstrated all too starkly, it seems that for some -- credit card solicitors, cellphone plan providers and, occasionally, mass murderers -- the modern American university is nothing more than a ready and convenient collection of nameless faces.