pt. 1. Entangled beginnings. Introduction : the science and ethics of mattering ; Meeting the universe halfway ; Diffractions : differences, contingencies, and entanglements that matter -- pt. 2. Intra-actions matter. Niels Bohr's philosophy-physics : quantum physics and the nature of knowledge and reality ; Agential realism : how material-discursive practices matter -- pt. 3. Entanglements and re(con)figurations. Getting real : technoscientific practices and the materialization of reality ; Spacetime re(con)figurings : naturalcultural forces and changing topologies of power ; Quantum entanglements : experimental metaphysics and the nature of nature ; The ontology of knowing, the intra-activity of becoming, and the ethics of mattering -- Appendix A. Cascade experiment / by Alice Fulton -- Appendix B. The uncertainty principle is not the basis of Bohr's complementarity -- Appendix C. Controversy concerning the relationship between Bohr's principle of complementarity and Heisenberg's uncertainty principle.