" ... Hogwarts
was founded over a thousand years ago -- the precise date is uncertain
-- by the four greatest witches and wizards of the age. The
four school Houses are named after them: Godric
and Salazar Slytherin.
They built this castle together, far from prying Muggle eyes, for it was
an age when magic was feared by common people, and witches and wizards
suffered much persecution.
"For a few years, the founders worked in harmony together, seeking out youngsters who showed signs of magic and bringing them to the castle to be educated. But then disagreements sprang up between them. A rift began to grow between Slytherin and the others. Slytherin wished to be more selective about the students admitted to Hogwarts. He believed that magical learning should be kept within all-magic families. He disliked taking students of Muggle parentage, believing them to be untrustworthy. After a while, there was a serious arguement on the subject between Slytherin and Gryffindor, and Slytherin left the school."
Since that time Hogwarts School of Witchcraft and Wizardry has been educating all magical talent, preparing its students for the magic of the future.