“Ave bossa nova similis bossa seneca” sums it up, I think.

Marxism is, in its pure form, has some quite nice theories, with only one major flaw: for Marxism to work, it would require human nature to be completely other than it has been experienced to be.

Or in other words: if people weren't tribalistic, hierarchical, pack-hunting arseholes, then Marxism might work. But then, if this were so, then Marxism wouldn't be needed.