It claims to be a science. Since sciences have laws, a failure in a law disproves the science. Marxism (and its quirky derivatives) is fundamentally flawed in many ways, never mind one, hence disproving the whole pile of poo.
Take these examples:
"Workers by brain, as well as by hand"
Yeah, right. That's Marx trying to save his own skin right there. Come the revolution, the intellectuals are going to get dragged out into the street and shot. If this isn't the case, why try to curry favour with the blue collar chumps? I grew up in a working class area where intellectuals got thumped.
"From each according to his ability"
Basic flaw - assumption that man is good. Why should Vaclav work hard if Leon, sleeping under the tractor, is going to get his share of Vaclav's goods? This is an incentive to be as lazy as possible and take what everyone else has got.
So you see, life isn't always serious. Ascribe to Gruntism and let your children run wild and free because, like the old sayings goes, let your children run wild and free.
[Note: this is a joke. Gruntboy accepts no liability for your lack of a sense of humour. I read Marx years ago and quickly put it away along with such other out-dated political tat as "Mein Kampf". Or will certain people soon be espousing Mr Adolf's views alongside Marx's? Hmmm?]