Capital Vol. 1 Chapter 10
The well-being of workers doesn't matter to capitalism. You'd think it would, because if workers can't get good rest and sustenance they won't be able consistently to do good work. But there's always an excess of potential workers, & if you run one into the ground you can hire another
Marx concludes that capitalists won't concern themselves with workers' well-being unless forced, & so workers must represent their interests through struggle