Why did the mathematical react to the cathedral children's comment about the happy prince?

The Mathematical was a man of math and science, he did not believe in things you cannot see.... he did not approve of dreams.

“How do you know?” said the Mathematical Master, “you have never seen one.”
“Ah! but we have, in our dreams,” answered the children; and the Mathematical Master frowned and looked very severe, for he did not approve of children dreaming.