Leo Apostel was born Antwerp in 1925. After the second World War he studied philosophy at the ULB in Brussels with philosopher of law and logician Chaïm Perelman. He got his M.A. at the ULB in Brussels in October 1948 with the thesis Questions sur l'Introspection. For another year he stayed here working as an assistant of Perelman.
In 1950-1951 Apostel is a CRB fellow at the University of Chicago with Rudolf Carnap and with Carl Gustav Hempel at Yale University. He promoted at the ULB in March 1953 with the dissertation "La Loi et les Causes". In 1955 he went to GenevaSwitzerland to study a work with Jean Piaget at the Centre International d'Epistémologie Génétique. Those experiences would influence the rest of his life.