In the first place, most libertarians are "subjectivists" in economics, that is, they believe that the utilities and costs of different individuals cannot be added or measured. Hence, the very concept of social costs and benefits is illegitimate. But, more importantly, most libertarians rest their case on moral principles, on a belief in the natural rights of every individual to his person or property. They therefore believe in the absolute immorality of aggressive violence, of invasion of those rights to person or property, regardless of which person or group commits such violence.