In his project of going beyond the “modern worldview,” Hiromatsu Wataru attached great importance to Ernst Mach’s philosophical thought and Einstein’s theory of relativity as challenging the premises of modern philosophy, which he characterized as substantialist and bound by the subject / object schema. This paper surveys Hiromatsu’s analysis of Mach’s phenomenalist element-monism, specifically his critique of Mach’s insufficient break with modern philosophy; his inquiry into Einstein’s relativity theory with a focus on its intersubjective cognitive structure; and the way he extends his views on these themes to a general ontological-epistemological theory of the “fourfold structure.” Finally, it examines questions about Hiromatsu’s arguments regarding the tension between the dimensions of synchronic structure and structuring movement. An earlier version of this paper can be found as “Philosophers” in John T. Blackmore, Itagaki Ryōichi, and Tanaka Setsuko, eds., Ernst Mach’s Influence Spreads, 425–76