At the base of Whitehead’s philosophy of organism is a vision of the solidarity of all final actualities. Each actuality is a discrete individual enjoying autonomous self-determination, yet each also requires all other actualities as essential components and partial determinants of its own nature. This vision of universal solidarity, Nobo demonstrates, is the fundamental metaphysical thesis whose truth the categories and principles of Whitehead’s philosophy were expressly designed to elucidate. The received interpretations of Whitehead’s thought, Nobo shows, have ignored the mutual relevance of the solidarity thesis and the organic categoreal scheme and, for that reason, have grossly misrepresented many of Whitehead’s most important metaphysical doctrines. Contending that the difficult tasks of interpreting and developing Whitehead’s metaphysics presuppose an understanding of the solidarity thesis, Nobo explores that thesis and the metaphysical categories and principles most relevant to its elucidation. In the process, he not only corrects many misinterpretations but also develops important metaphysical doctrines that Whitehead neglected to make sufficiently explicit in his published writings. It is precisely in terms of the neglected doctrine of eternal extensive continuity, Nobo demonstrates, that the more puzzling aspects of the solidarity thesis are satisfactorily explained. He then shows that the extensional solidarity of all final actualities is an essential ingredient of the generalized conception of experience on which Whitehead builds his ontology, cosmology, and epistemology.