This project is concerned with the theoretical foundations of our biological understanding of the nature of organisms. Since Descartes, the dominant school of biological thought regarding the nature of life has been mechanicism, which conceives organisms in analogy with machines. Guided by this conception, mechanicism understands biological wholes in terms of the activities of their component parts; it characterizes the properties of living systems from the bottom up in increasing levels of complexity; it emphasizes the causal and material attributes of organisms over their organizational and teleological features; and it vindicates the appeal to explanatory reductionism in the study of organisms. Although mechanicism has played a fundamental role in the historical development of biology, and indeed still underlies many areas of biological inquiry today, there is a growing awareness in the biological community that it no longer provides an adequate theoretical framework in which to make sense of the most recent advances in our understanding of organisms. The project attempts to lay the philosophical groundwork for the articulation of a biologically compelling theoretical alternative to the mechanistic conception of life. To do so, the project draws on the rich, yet largely neglected anti-mechanistic tradition in biology. In the early twentieth century, a number of theoretical biologists, including J. S. Haldane, J. H. Woodger, and L. v. Bertalanffy, used the classic vitalistic critiques of mechanicism to develop a new naturalistic theory of the organism called organicism. Organicism takes the teleological self-organizing nature of living systems as the hallmark of their ontological distinctiveness, and by implication regards biology as an autonomous science possessing its own theoretical principles distinct from those of physics and chemistry. Although organicism flourished for a brief period, it was eclipsed by the molecular revolution that swept biology in the 1950s, which consolidated mechanicism for the rest of the century. The main contention of the project is that organicism possesses the necessary intellectual resources to supplant mechanicism as a general theory of living systems. Thus, the project aims to use the pioneering ideas of several early twentieth century theoretical biologists as a springboard to formulate an organicist philosophy of the organism capable of coming to terms with the latest empirical findings of biology. The project is not only concerned with biological ontology, as the organicist conception of life also has important implications for the study of living systems and our understanding of biology as a science.