 After WWI David Hilbert (1862–1943), who had at first adopted the algebraic approach, picked up on the approach of Principia, and the algebra of logic fell out of favor. However, in 1941, Tarski treated relation algebras as an equationally defined class. Such a class has many models besides the collection of all binary relations on a given universe that was considered in the 1800s, just as there are many Boolean algebras besides the power set Boolean algebras studied in the 1800s. In the years 1948–1952 Tarski, along with his students Chin and Thompson, created cylindric algebras as an algebraic logic companion to first-order logic, and in 1956 Paul Halmos (1916–2006) introduced polyadic algebras for the same purpose. As Halmos (1956 b, c and d) noted, these new algebraic logics tended to focus on studying the extent to which they captured first-order logic and on their universal algebraic aspects such as axiomatizations and structure theorems, but offered little insight into the nature of the first-order logic which inspired their creation.