 First, he deploys the corpuscularian hypothesis against the Scholastics to argue that ‘borderline cases’ create a serious challenge to their doctrine of substantial forms, but which are allowed for, and even explained by, corpuscularian principles. By his lights, if we assume that nature produces substances by imposing a substantial form (what Locke mockingly calls ‘moulds and forms’), or species essence onto them, then the existence of entities that either fit into more than one species, or do not fit into any single species, shows us at least that not all beings are produced according to natural moulds and forms: