Over the course of the twentieth century, the notion of the systematicity of thought has acquired a much narrower meaning than it used to carry for much of its history. The so-called “systematicity debate” that has dominated the philosophy of language, cognitive science, and AI research over the last thirty years understands the systematicity of thought in terms of the compositionality of thought. But there is an older, broader, and more demanding notion of systematicity that is now increasingly relevant again. (...) To recover this notion from under the shadow of the systematicity debate, I distinguish between (i) the systematicity of thinkable contents, (ii) the systematicity of thinking, and (iii) the ideal of systematic thought. I then deploy this distinction to critically evaluate Fodor’s systematicity-based argument for the language of thought hypothesis before recovering the notion of the systematicity of thought as a regulative ideal, which has historically shaped our understanding of what it means for thought to be rational, authoritative, and scientific. To assess how much systematicity we need from AI models, I argue that we must look to the rationales for systematizing thought. To this end, I recover five such rationales from the history of philosophy and identify five functions served by systematization. Finally, I show how these can be used to arrive at a dynamic understanding of the need to systematize thought that can tell us what kind of systematicity is called for and when. (shrink)