Full Idea
Some philosophers (e.g Plato) begin with an account of reality, and then appended an account of how we can know it, ..but Descartes turned the tables, insisting that we must first decide what we can know.
Gist of Idea
Most philosophers start with reality and then examine knowledge; Descartes put the study of knowledge first
Source
Keith Lehrer (Theory of Knowledge (2nd edn) [2000], I p.2)
Book Reference
Lehrer,Keith: 'Theory of Knowledge' [Westview 2000], p.2