At first the four founders worked together, bringing together young witches and wizards to teach them the magical arts. However, one founder – Salazar Slytherin – believed that only those from all-magic families should receive their education at Hogwarts. Ravenclaw, along with the other two founders – Helga Hufflepuff and Godric Gryffindor – welcomed Muggle-borns into the school, which prompted Slytherin to leave.
The Sorting Hat reveals in its song that Rowena Ravenclaw wanted the cleverest students for her house. Along with the other founders, Rowena helped give the Sorting Hat brains to allow it to sort the students itself after they were gone.