Marx's method was to extract the rational kernel, do a dialectical critique or sublate ( overcome , BUT ALSO PRESERVE THE RATIONAL KERNEL) of those he critiqued . The most famous example is preserving the revolutionary rational kernel of Hegel while discarding the idealist husk. However, doing that didn't mean that other non-Marxists thinking is put on the same level with the essential principles of Marxism ( see Lenin for an essential summary) IN A MARXIST THEORY discussion. Marxism is not eclectic , but dialectic.