In the philosophy of perception, the theory of sense data was a popular view held in the early 20th century by philosophers such as Bertrand Russell, C. D. Broad, H. H. Price, A.J. Ayer, and G.E. Moore. Sense data are supposedly mind-dependent objects whose existence and properties are known directly to us in perception. They are supposed to be unanalyzed experiences inside the mind, which appear to subsequent more advanced mental operations exactly as they are.