Unspoken assumptions of safety underlie our everyday life. Civilization rests on restraints embodied in our most fundamental moral codes. When we go to work in the morning, we don’t expect that in our absence our house will be robbed. When we go out to the movies, for all the mayhem we may see on the screen, we don’t expect gunfire to break out in the theater.

The killings in Aurora, Colo., early Friday morning were all the more shocking because of their seeming randomness, the absence of purpose, the senselessness. They take their place in a long list of incidents that represent, more than anything else, a repudiation of those rules of civilization, the unspoken assumptions that allow us to live together in human society.

President Obama said that the killings reminded us that we are a family, the human family, bound together by a deeply rooted sense of compassion that allows us to go about our business each day in an atmosphere of respect. The ordinary give and take of daily life sometimes puts to the test our compassion and challenges the sense of respect that we rely on. But when these fundamental principles of human conduct are trammeled as they were in Colorado, we see the ties that bind us more clearly. And we see how strong they are.

James Holmes joins the ranks of other mass murderers who may have espoused one purpose or another but whose actions were fundamentally an attack on the idea of human connection, human responsibility. In this country, some of the most memorable recent attacks occurred in Tucson, Ariz., in 2011; at Fort Hood, Texas, in 2009; at Virginia Tech University in 2007; and at Columbine High School, Colorado, in 1999. Outside the United States mass murders have occurred in Norway, Germany, Finland, Scotland, Australia.

Ordinary moral condemnation does not seem adequate to these actions. Toward the shooters, we can hurl the most extreme moral denunciations, and they do not capture the full awfulness of the crimes. Partly, it’s because the crimes are so impersonal and random. The arbitrariness of the crimes gives them a chilling quality. It is as if a seed of evil had escaped from the plant of Nazism, an ultimate form of nihilism that does not require justification and thrives on an absence of conscience.

We know, of course, that the killers in these crimes are, if not technically insane, then alienated from the bonds of human empathy. Sociopaths have been identified as people who for whatever reason seem to lack a conscience or a sense of human connection. Not every sociopath opens fire in a movie theater. But it appears there is a flaw in the fabric of humanity that is perennially with us. In a more brutish time, a sociopath may have hired himself out as an assassin or an Indian killer or a slave driver. Who knows what was lurking in the mind of James Holmes?

Of course, America provides ample opportunity for people who want to do harm with guns. It is part of the pathology of America that its love of firearms allows criminals to arm themselves with impunity. That is a debate that will be stirred up by this incident as it is stirred up by every similar incident. But nothing will be done because politicians are afraid of gun lovers.

The fact is that we take comfort at times like these from the sense of human solidarity that becomes evident and from a renewed determination to cherish life over death, compassion over cruelty. We set aside the petty differences that preoccupy us in ordinary times and express our gratitude for the human family of which we are a part and the glorious life that we have been given to experience.