To identify a causal effect is to reduce a causal expression to a purely statistical expression. In this chapter, that means to reduce an expression from one that uses potential outcome notation to one that uses only statistical notation such as 𝑇 , 𝑋 , 𝑌 , expectations, and conditioning. This means that we can calculate the causal effect from just the observational distribution 𝑃(𝑋, 𝑇, 𝑌)