In a medium soup pot, heat butter. Add leeks and celery. Sauté for 5 minutes over medium-high heat, until they begin to soften.
Add raw pumpkin cubes, broth, sage, salt and pepper. Cover and bring to a simmer. Reduce heat to medium-low and simmer for 10 minutes, until the pumpkin is tender.
In a small bowl, whisk together milk and flour until smooth. Add the milk mixture to the soup along with the corn. Return the soup to just below a simmer and continue to cook for 20 minutes, stirring occasionally. (The more you stir the chowder, the more the pumpkin will break apart and thicken the soup.)