Chandran Kukathas argues that Hayek's defence of liberalism is unsuccessful because it rests on presuppositions that are incompatible. The unresolved dilemma of his political philosophy is how to mount a systematic defence of liberalism if one emphasizes the limited capacity of reason. Norman P. Barry similarly notes that the "critical rationalism" in Hayek's writings appears incompatible with "a certain kind of fatalism, that we must wait for evolution to pronounce its verdict". Milton Friedman and Anna Schwartz argue that the element of paradox exists in the views of Hayek. Noting Hayek's vigorous defense of "invisible hand" evolution that Hayek claimed has created better economic institutions than could be created by rational design, Friedman pointed out the irony that Hayek was then proposing to replace the monetary system thus created with a deliberate construct of his own design. John N. Gray summarized this view as "his scheme for an ultra-liberal constitution was a prototypical version of the philosophy he had attacked". Bruce Caldwell wrote that "f one is judging his work against the standard of whether he provided a finished political philosophy, Hayek clearly did not succeed", although he thinks that "economists may find Hayek's political writings useful".