The complex and difficult problem of causality is central to our understanding of nutrition research. A cause is defined as "that factor which is possible or convenient for us to alter in order to produce or prevent an effect. This concept contains two components: production of an effect and an understanding of its mechanisms." To understand current concepts of causality, it is helpful to briefly review historical thinking about it. Aristotle believed that bodies in motion required constant force (efficient cause) to keep them moving, that the seed contained the adult (teleological cause). After more than 2,000 years, Newton overturned Aristotle in physics with the concept of inertia. Hume further advanced our understanding by postulating that our notion of causality depends on well-documented associations. Partially correct, Kant believed the mind (brain) imposes notions of time, extension, and causality on nature.