While it was not until the modern era that epistemology was first recognized as a distinct philosophical discipline which addresses a well-defined set of questions, almost every major historical philosopher has considered questions about what we know and how we know it. Among the Ancient Greek philosophers, Plato distinguished between inquiry regarding what we know and inquiry regarding what exists, particularly in the "Republic", the "Theaetetus", and the "Meno". A number of important epistemological concerns also appeared in the works of Aristotle.