Don’t worry, I only read On Materialism this year and would have been clueless about this if you’d asked last year :D. I’m still absorbing it and the various follow-ups, so hardly an expert or authority.

I’ve been suspicious of humanism-itself for a lot longer, though. As with everything, it becomes watered down and repackaged over time, until you end up with pithy statements like “good without god”, etc, and as a general statement of anti-religion, fine, sure, that’s marxism-compatible, materiaism-compatible, etc. But in this form, it’s also unnecessary to synthesise it into marxism at all, since what you end up with is not “marxist-humanism”, it’s just “marxism”. The only reason to add “-humanism” to the end would be marketing, branding, that kind of thing.

What’s interesting to dig into is where humanism and marxism don’t overlap, whether that’s just addressing different things, or actually conflicting with each other. That’s where a synthesis of the two has tricky questions to answer, and maybe - just maybe - value to bring. That kind of effort will use a much more in-depth and rigorous understanding of what humanism is, with jumping-off points like the amsterdam declaration, etc.

What did marxist-humanists come up with when doing this? I’m as in the dark as you as to specifics, but https://en.wikipedia.org/wiki/Marxist_humanism briefly say:

Marxist humanists reject an understanding of society based on natural science, asserting the centrality and distinctiveness of people and society.

So this basically says “historical materialism is false”, and it comes about because humanists believe individual human free will is supreme and, somehow, the aggregate effect of humans having free will cannot be investigated scientifically.

Marxist humanism views Marxist theory as not primarily scientific but philosophical. Social science is not another natural science and people and society are not instantiations of universal natural processes. Rather, people are subjects – centers of consciousness and values – and science is an embedded part of the totalizing perspective of humanist philosophy.

So much for “scientific socialism”, I suppose. My best summary of this would be “science doesn’t work on people, sike”. Forgive me my skepticism.

Echoing the inheritance of Marx’s thought from German Idealism, Marxist humanism holds that reality does not exist independently of human knowledge, but is partly constituted by it

I’ll get my coat.

(edit: I didn’t say explicitly, but each of these three examples is chock-full of idealism, which is to say, “metaphysical perspectives which assert that reality is indistinguishable and inseparable from perception and understanding; that reality is a mental construct closely connected to ideas”. What marxist-humanism takes from humanism, in these passages, is idealism, and the “synthesis” is to discard materialism and replace it with that idealism)