This paper presents what I will call Arendt's genealogy of thinking. My purpose in doing so is to strengthen Arendt's critique of thoughtlessness which I believe is both a powerful, but underappreciated analytic tool and a consistent, but under-examined thread that occurs throughout Arendt's oeuvre. To do so I revisit her phenomenology of thinking and the distinction between thinking and cognition she introduces in her last, unfinished work, The Life of the Mind. When read alongside the genealogy of action in The Human Condition to create a parallel genealogy of thinking, we can begin to see that the implications of this distinction between thinking and cognition are radical, challenging, and deeply important.