Full Idea
When I speak of 'simples' I am speaking of something not experienced as such, but known only inferentially as the limits of analysis.
Gist of Idea
'Simples' are not experienced, but are inferred at the limits of analysis
Source
Bertrand Russell (Logical Atomism [1924], p.158)
Book Reference
Russell,Bertrand: 'Russell's Logical Atomism', ed/tr. Pears,David [Fontana 1972], p.158
A Reaction
He claims that the simples are 'known', so he does not mean purely theoretical entities. They have something like the status of quarks in physics, whose existence is inferred from experience.