In the 6th book of his "Nicomachean Ethics", Plato's student and friend Aristotle famously distinguished between two intellectual virtues: "sophia" (wisdom) and "phronesis", and described the relationship between them and other intellectual virtues. "Sophia" is a combination of nous, the ability to discern reality, and "epistēmē", a type of knowledge which is logically built up and teachable, and which is sometimes equated with science. This involves reasoning concerning universal truths. "Phronesis" involves not only the ability to decide how to achieve a certain end, but also the ability to reflect upon and determine good ends consistent with the aim of living well overall.