At common law, the landlord was not prohibited from discriminating in selecting his tenants. He could refuse to rent for any reason or for no reason at all. The building was his real property and he had the right to exclude others from his land.
But in 1968, Congress enacted the Fair Housing Act (as amended from time to time), which is set forth on p.459 of the casebook.
How much liberty should landlords have in selecting tenants? Which categories of persons should be protected againt discrimination?
Race?
Religion?
Gender?
Sexual Orientation?
Families with children?
People who own pets?
Political or ideological status?
Criminal convictions?