In early modern European natural philosophy and medicine, scholars encountered the problem of the “formless birth” in their studies into generation, alongside “monstrous” and “perfect” births. Such formless births included the hen’s egg, the unformed bear cub, and the human false conception – said to be shapeless lumps of moving flesh – and these types of conceptions influenced how natural philosophers, like William Harvey and Jan Baptiste van Lamzweerde, approached experiments on, or explanations of, generation. This article suggests that the (...) history of these unformed births provides fresh insights into how early modern scholars “thought” with formlessness. That is, early modern scholars drew on a longstanding discourse of shapeless conceptions to address the mechanisms of formless generation, “true” generation, as well as to understand other unformed or interstitial creatures in nature. The article argues that this history of thinking with formless births contributes to our picture of the significant role that unnatural births, such as “monsters”, played in early modern natural philosophy. (shrink)