Rationality, Ability to Pay, Need

Marx' formula, 'From each according to one's ability, to each according to one's need' has no obvious derivation from Dialectical Materialist Rationalism.  Nevertheless, it is perhaps potentially more subversive of Capitalism from anything derived from that principle.  Now, while it may seem Utopian and unrealistic, there are actually not too uncommon illustrations of one instantiation of it.  The instantiation is a simple purchase, in which one pays what one can and takes what one needs, a practice not too unusual in contexts such as co-ops.  The significance of such a transaction is that it defies all concepts of 'rational' behavior that are ingredient in Capitalist theory, beginning with any thesis that Profit-seeking is the fundamental drive of human behavior.  So, implicit in Marx' formula is an alternative to Capitalism that may also be an alternative to Marxist Socialism.