The chief defect of all hitherto existing materialism – that of Feuerbach included – is that the thing, reality, sensuousness, is conceived only in the form of the object.
Does Marx allege any other mistakes, and is he right to?
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Marx objected to aspects of Helvetius’ materialism, Lucretius’ materialism, Locke’s materialism, Epicurus’ materialism, Kant’s materialism, etc.
The so-called “chief defect” summarizes the problem with all noteworthy progressive philosophies heretofore existing — that they were speculative, and shirked from the practico-theoretical.
To expand your understanding of his critique, read the Theses in full, which is propaedeutic to the German Ideology that best addresses your question.
Keep in mind that Marx’s materialism is a controversial subject. Marx himself didn’t value his early epistemological works later in life (preferring to them the Critique of Political-Economy and the preliminary notes thereto collected under the title, Grundrisse).
Read Feuerbach's Lectures on the Essence of Religion. Or if you want the hard version, Essence of Christianity.
Then read Marx's theses on Feuerbach. It will clarify Marx's criticisms. You shall see how much Marx took from Feuerbach.