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Owning the means of production called "knowledge"
These workers did not own the means of production. They could only produce by borrowing increasingly expensive machines from capitalists.
Knowledge workers also require tools for production.
Moreover, capital investment in tools that knowledge workers need is likely to be much higher than that for any physical laborer's tools. However, such capital investment cannot be productive unless it is accompanied by the means of production that knowledge workers own, namely knowledge. In addition, investment in education for knowledge workers far exceeds that for physical laborers.