We live and we think inside a world of things made and found. Still, psychological science has shown little interest in understanding the exact nature of the relation between cognition and material culture. As a result, the diachronic influence and transformative potential of things in human mental life remains little understood. Most psychologists would see things as external and passive: the lifeless objects of human consciousness, perception, and memory. On the contrary, my main argument in this article is that things matter to human psychology and should be taken seriously. Although things usually pass unnoticed, they are anything but trivial. Things have a special place in human cognitive life and evolution. We think “with” and “through” things, not simply “about” things. In that sense, things occupy the middle space in between what are usually referred to as mind and matter. Material-engagement theory provides a way to describe and study that middle space where brain, body, and culture are conflated.