The function of language in human cognition is mysterious. Which of our thoughts could we think in the absence of language? Which of our thoughts, if any, require language? If some of our thoughts indeed require language, what does language do that is so special? In the first part of the talk, I will present a series of studies exploring the limits of thought without language, using evidence from human spatial representation and navigation. In the second part of the talk, I will lay out some new ideas about how referential properties of language might support otherwise unthinkable thoughts. I will end with concrete predictions about spatial behavior, based on this theoretical model, that we are currently testing in my lab.