Based on the song of the Sorting Hat itself one year, it seemed that in the beginning, each of the founding wizards had their own preference on who to accept (except Helga Hufflepuff, who would accept any witch or wizard who wanted to learn).

Excerpt: (from Harry Potter Wiki)

Said Slytherin, "We'll teach just those

Whose ancestry's purest."

Said Ravenclaw, "We'll teach those whose

Intelligence is surest."

Said Gryffindor, "We'll teach all those

With brave deeds to their name."

Said Hufflepuff, "I'll teach the lot

And treat them just the same."

The hat also recognized the inherent error in dividing the students:

And now the Sorting Hat is here

and you all know the score:

I sort you into Houses

because that is what I'm for.

But this year I'll go further,

listen closely to my song:

though condemned I am to split you

still I worry that it's wrong,

Though I must fulfill my duty

and must quarter every year

still I wonder whether sorting

may not bring the end I fear.

Based on the song that the Sorting Hat itself sang one of the years, it seemed that in the beginning, each of the founding wizards had their own preference on who to accept (except Helga Hufflepuff, who would accept any witch or wizard who wanted to learn).

Excerpt: (from Harry Potter Wikia)

Said Slytherin, "We'll teach just those

Whose ancestry's purest."

Said Ravenclaw, "We'll teach those whose

Intelligence is surest."

Said Gryffindor, "We'll teach all those

With brave deeds to their name."

Said Hufflepuff, "I'll teach the lot

And treat them just the same."

The hat also recognized the inherent error in dividing the students:

And now the Sorting Hat is here

and you all know the score:

I sort you into Houses

because that is what I'm for.

But this year I'll go further,

listen closely to my song:

though condemned I am to split you

still I worry that it's wrong,

Though I must fulfill my duty

and must quarter every year

still I wonder whether sorting

may not bring the end I fear.