 A theory of development should explain the core phenomena of growth, cell differentiation, and morphogenesis, which together transform an egg into a mature organism. However, beyond this rough consensus uncertainty abounds. Exactly what entities undergo development, and what are the process’ temporal, spatial, and functional boundaries? These questions are debated in biology and philosophy (Bonner 1974; Pradeu et al. 2011). Also unsettled are the nature and significance of scientific theories. Whether a general theory of development is possible or desirable hinges in part on how “theory” is defined. Yet there is no philosophical consensus to fall back on. Furthermore, developmental biology is not a traditionally theoretical field (in contrast to, say, evolutionary biology; see the entry on  developmental biology).  A recent edited collection, Towards a Theory of Development, reveals a variety of viewpoints and frameworks, ranging from skepticism about any such theory, to specific proposals influenced by physics, mathematics, biochemistry, systems biology, and more (Minelli &amp; Pradeu 2014a). Broadly speaking, however,