A teaching of the Buddha about four categories of things to be mindful of (keep in mind, focus on) in order to progress on the spiritual path to liberation: the body, feelings (pleasant, unpleasant, neutral), mind (states), and mental qualities (in particular those referred to in the Buddha’s teachings as being either beneficial or problematic in practice). A classic source on the Four Foundations of Mindfulness (also know as the four “establishings” of mindfulness) is the Satipatthana Sutta.