According to the Stoics, our cognitive systems are so designed that certain notions arise in every individual and constitute the “common conceptions” which serve as a touchstone (criterion) in argument. This paper shows how Neoplatonist philosophers from Plotinus to Iamblichus integrate the stoic view into their accounts of knowledge. More precisely, Neoplatonist philosophers regard common conceptions as ordinary concepts which arise naturally in humans and reflect basic features of reality (it is controversial whether common conceptions involve inborn cognitive contents). Plotinus and Porphyry furthermore suggest that common conceptions are the same for all humans, while disagreement arises through philosophical enquiry: it is therefore crucial to select those philosophical doctrines which develop common conception adequately and do not conflict with them.  Plotinus’ discussion of time in 3.7, Porphyry’s account of knowledge and definition, Porphyry’s and Iamblichus’ discussions about theology provide interesting and under-investigated evidence about these aspects of Neoplatonist epistemology.