Trust is difficult to define. Instead of doing so, I propose that the best way to understand the concept is through a genealogical account. I show how a root notion of trust arises out of some basic features of what it is for humans to live socially, in which we rely on others to act cooperatively. I explore how this concept acquires resonances of hope and threat, and how we analogically apply this in related but different contexts. The genealogical account explains both why the notion has such value for us and why it is difficult to define.