This document discusses recursive and recursively enumerable sets. It defines a procedure as a finite sequence of instructions that can be mechanically carried out for any input, and an algorithm as a procedure that terminates after a finite number of steps. It states that a set is recursive if there is an algorithm to determine if an element belongs to the set, and a set is recursively enumerable if there is a procedure to determine membership that may not terminate for all inputs. It also notes that context sensitive languages are recursive.