The ninth amendment was included to the Bill of Rights to warrant that the maxim expressio unius est exclusio alterius would not be used at a later time to refute fundamental rights simply because they were not explicitly enumerated in the Constitution.
The record of the Constitution reveals the function of the Ninth amendment and the Founders' intent: to defend what constitutional lawyers describe as the unenumerated rights. Unenumerated rights include, as an example, the right to privacy. In the USA of today, unenumerated rights include for freedoms like a woman's right to abortion.
The Ninth amendment goes so far beneath the radar that it has hardly ever been mentioned even by the Supreme Court. That is why sometimes it is called the Silent Ninth amendment.
