Humans' disposition for rational learning, planning, and decision-making is unparalleled in the natural world. Foundational views in the cognitive sciences hold that to learn, think and talk as we do, we rely upon a "language of thought" – a capacity for framing ideas in abstract logical structures and for drawing the inferences they support. Yet, we know little about the foundations of logical thought in the ontogeny of the mind: are learning, education, or the mastery of language required for logical cognition? In this talk, I will present my attempt to answer this question. First, I discuss a developmental primitive of logical reasoning: infants make inferences by contrasting and eliminating alternatives. Through a series of studies, I will examine the nature of this preverbal logical capacity and its function in knowledge acquisition. Next, I will present newer work investigating the breadth of infants' and children's uncharted logical resources: (i) the presence of other fundamental logical representations, (ii) their abstractness and domain-generality, and (iii) the capacity to integrate distinct logical operations. My goal will be to share with you works that aim to shed light on the developmental foundations upon which thought is built and made possible.