While Marx accepted Hegel's dialectic process, he denied Hegel's conclusion that this dialectic ends in freedom that develops at the level of society in the form of Spirit.
What is Marx's argument against this conclusion?
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One does not have to agree to a conclusion if he agrees to all the facts leading to it.
Example: you can have 5 symptoms and a nurse can maybe formulate a diagnostic bases on that. But the doctor with many years of experience may independently check that all 5 symptoms exists and manifest as described by the nurse but he may set a totally different diagnostic.
Hegel himself admitted that his dialectical method was part of a philosophical tradition dating back to Plato but he criticized Plato’s version of dialectics. Hegel argued that Plato’s dialectics deals only with limited philosophical claims and is unable to get beyond skepticism or nothingness. According to the logic of a classic reduction to absurd argument, if the premises of an argument lead to a contradiction, we must conclude that the premises are false. That leaves us with practically nothing.
In turn, Karl Marx presented his own dialectic method, which he claims to be a direct opposite of Hegel's method.
Direct quote from Marx in the paper "Afterword":
My dialectic method is not only different from the Hegelian, but is its direct opposite. To Hegel, the life-process of the human brain, i.e. the process of thinking, which, under the name of 'the Idea', he even transforms into an independent subject, is the demiurgos of the real world, and the real world is only the external, phenomenal form of 'the Idea'. With me, on the contrary, the ideal is nothing else than the material world reflected by the human mind, and translated into forms of thought.
And that's the direct answer to your question, given by Marx himself.
We have to remember that Karl Marx was an Atheist; in today's culture he would be referred to as a "Secular Humanist" or "Pan-Materialist"-(I would not use the trite sounding word, "Progressive", which Marx himself may not have necessarily welcomed).
For Marx, the Hegelian-(and even Platonic) notions of "Ideas" or Grand "Forms", was a wrong headed approach towards understanding the nature of Reality and Materialism. Marx held the "opposite" view that if there was and is no God, then there was and is no "Spirit"-(neither the angelic spirit, nor The Greater "Spirit"/Force that is historically Purposeful).
Essentially, Marx was a Pan-Realist, whereas Hegel, was more of a Transcendentalist.