Not bodies produce sensations, but element-complexes (sensation-complexes) constitute the bodies. When the physicist considers the bodies as the permanent reality, the elements' as the transient appearance, he does not realise that all bodies' are only mental symbols for element-complexes (sensation-complexes)

p. 23, as quoted in Lenin as Philosopher: A Critical Examination of the Philosophical Basis of Leninism (1948) by Anton Pannekoek, p. 33 - 20th century - The Analysis of Sensations, (1902)