Mach's phenomenalism and neutral monism inevitably lead to idealism and solipsism

In his lecture on metaphysics as in his paper “Leib und Seele,” Stumpf’s main target is Mach’s phenomenalism and neutral monism, which amounts to reducing physical objects and subjectivity to functional relations between sensations (or elements) and inevitably leads, according to Stumpf, to idealism and solipsism. Stumpf published in 1919 a very interesting study on Spinoza’s parallelism in which he proposes an original and unorthodox interpretation of Spinoza’s Ethics. He argues that Spinoza’s