 In 1911–1912, Russell returned to a project he had started soon after the break from Idealism, an investigation of the concept of matter. A need to first secure the foundations of mathematics had diverted him from this project (and resulted in Principia). Now, from 1912 on, he approached the analysis of matter from the perspectives of epistemology and ontology, investigating our knowledge and conception of the fundamental constituents of the external world. The main events concerning sense data proper run from the Problems of Philosophy (1912) to the Analysis of Mind (1921), although the legacy of sense data extends further.