LOUIS ALTHUSSER, a brilliant French philosopher famous for writing eloquently about the nature of disobedience for many years before suddenly and inexplicably shooting his wife, is the author of a little essay that reminds me very much of a hacker who named himself Fyodor. The essay, one of Althusser's masterpieces, is called "Ideology and Ideological State Apparatuses." Fyodor's masterpiece is a software program called nmap.

I had the pleasure of watching Fyodor conduct an informal presentation about nmap at the San Francisco OpenBSD User Group (SFOBUG). With his generous intelligence and humble plainspokenness, Fyodor is a geek's geek. Since he published an article about it in Phrack six years ago, nmap has grown from a small-scale program to a powerful precision tool called a port scanner that's used for investigating whether a computer is running any programs that might make it vulnerable to attack or exploitation.

The difficulty with nmap is that it's a great tool to use if you want to break into a computer network. The difference between a "good" nmap user and a "bad" nmap user is that the good one wants to fix the vulnerabilities she or he finds and the bad one wants to exploit them. Several members of SFOBUG reported that nmap's naughty reputation has gotten them into numerous scrapes – some of them said their ISP accounts had been shut down by administrators who found nmap in their directories and accused them of possessing "dangerous hacker tools."

In 2000 an engineer named Scott Moulton was brought up on criminal charges in Georgia for using nmap to scan a network he'd been contracted to maintain. A judge ruled that port scanning was legal under state computer crime laws, but Moulton still faces criminal charges from another company he scanned.

The ambiguity nmap confers on its users reminded me of Althusser's essay and the concept of "interpellation" he elaborates in it. Interpellation refers to the way people come to know themselves through a curious process of misrecognition. Althusser's argument rests on the notion that people learn who they are from the way institutions and other people define them. A person who considers himself a quick-witted computer programmer, interpellated by propaganda from the federal government or ISP administrators, might suddenly misrecognize himself as a criminal. In a famous passage about this, Althusser describes a person walking along the street suddenly hailed by a police officer who screams, "TRESPASSER!" The person cannot help turning around as if his own name is being called; by responding to the hail, he seems to say, "Yes, I am a trespasser." In that moment of misrecognition, he is given a new name, a new identity. The police officer has made him see himself, if only briefly, as a trespasser.

The word interpellation comes from the Latinate "inter" (between) and "appello" (to name). In a literal sense, the philosopher is describing how humans' names for themselves arise out of interactions between people: parents name their children, police officers name trespassers, and judges name nmap users bad or good. Regardless of your intentions or desires, you cannot always control the names you are given. And those names define you in ways you never intended; indeed, they can even confine you, as Moulton discovered when he was brought up on criminal charges for using nmap.

Names carry political freight. That's why it's crucial for nmap users – and others like them – to fight back when ISPs refuse to offer them service because they misrecognize a tool that most people use to defend computers against intrusions or to learn about the vulnerabilities in their networks. When the authorities call us trespassers, we should refuse to acknowledge that name. We are not criminals.

Talking to Fyodor in another context, with Althusser still on my mind, I asked him, "Do you want to tell me your real name?" Fyodor is the hacker handle he picked out for himself as a teenager. It isn't as if the name his parents gave him is a huge secret, although I don't know it. I was simply curious about whether he wanted to tell me, which isn't the same thing as finding it out.

"No," he said with a smile.