gold and gold for goods." The following is an interpretation of Heraclitus's concepts into modern terms by Nicholas Rescher. "...reality is not a constellation of things at all, but one of processes. The fundamental "stuff" of the world is not material substance, but volatile flux, namely "fire", and all things are versions thereof (puros tropai). Process is fundamental: the river is not an object, but a continuing flow; the sun is not a thing, but an enduring fire. Everything is a matter of process, of activity, of change (panta rhei)." An early expression of this viewpoint is in Heraclitus's fragments. He posits strife, ἡ ἔρις (strife, conflict), as the underlying basis of all reality defined by change. The balance and opposition in strife were the foundations of change and stability in the flux of existence. Nietzsche and Kierkegaard In his written works, Friedrich Nietzsche proposed what has been regarded as a philosophy of becoming that encompasses a "naturalistic doctrine intended to counter the metaphysical preoccupation with being", and a theory of "the incessant shift of perspectives and interpretations in a world that lacks a grounding essence". Søren Kierkegaard posed questions of individual becoming in Christianity which were opposed to the ancient Greek philosophers' focus on the indifferent becoming of the cosmos. However, he established as much of a focus on aporia as Heraclitus and others previously had, such as in his concept of the leap of faith which marks an individual becoming. As well as this, Kierkegaard opposed his philosophy to Hegel's system of philosophy approaching becoming and difference for what he saw as a "dialectical conflation of becoming and rationality", making the system take upon the same trait of motionlessness as Parmenides' system. Twentieth century In the early twentieth century, the philosophy of mathematics was undertaken to develop mathematics as an airtight, axiomatic system in which every truth could be derived logically from a set of axioms. In the foundations of mathematics, this project is variously understood as logicism or as part of the formalist program of David Hilbert. Alfred North Whitehead and Bertrand Russell attempted to complete, or at least facilitate, this program with their seminal book Principia Mathematica, which purported to build a logically consistent set theory on which to found mathematics. After this, Whitehead extended his interest to natural science, which he held needed a deeper philosophical basis. He intuited that natural science was struggling to