"Moving on! I now introduce a new key definition, that of conditional probability:"
P(X◁Y) =def P(X & Y) / P(Y)
"For example, in the case of Int 15 wizards who make 5th-circle:"
P(INT 15) ~ 0.01
P(INT 15 & 5th-circle) ~ 0.0002
P(5th-circle ◁ INT 15) ~ 0.02 = 2/100
"If we start with 100,000 Chelish citizens, there should be around 1000 of them with INT 15, and then 20 of those who become 5th-circle wizards according to the statistics I totally made up this morning, and so 2% of people become 5th-circle wizards conditional on them having INT 15."
"The symbol is easy to remember because you can imagine that, on the right side, it shows a wide pool of people with INT 15, diminishing on the left side to a narrower pool of people who have INT 15 and are 5th-circle wizards."