 This is often taken to be the definition of conditional probability, although it should be emphasized that this is a technical usage of the term that may not align perfectly with a pretheoretical concept that we might have (see Hájek, 2003). We recognize it in locutions such as “the probability that the die lands 1, given that it lands odd, is 1/3”, or “the probability that it will rain tomorrow, given that there are dark clouds in the sky tomorrow morning, is high”. It is the concept of the probability of something given or in the light of some piece of evidence or information. Indeed, some authors take conditional probability to be the primitive notion, and axiomatize it directly (e.g. Popper 1959b, Rényi 1970, van Fraassen 1976, Spohn 1986, and Roeper and Leblanc 1999).