Throughout his career, Piaget rejected the adequacy of random trial and error, or variation and selection models. Instead, he argued that teleonomic autoregulations were necessary to account for the facts of evolution and development. This purported necessity of teleonomy has been a controversial and generally rejected aspect of his model – especially in its evolutionary version. Necessary teleonomy, however, is not an isolated part of Piaget’s thinking, but is instead deeply motivated by two central forces throughout Piaget’s oeuvre: a complex of assumptions organized around his structuralist assumption concerning the nature of knowledge, and the centrally important epistemological problem of logical necessity. Structuralism, however, is shown to be a seriously flawed foundation for Piaget’s epistemology, and to be at the center of a number of inadequate and erroneous positions within Piaget’s writings – positions concerning epistemology, evolution, and even necessity itself. An alternative conception of knowledge is outlined – interactivism – that offers a corresponding alternative approach to necessity.