Is the unstated central thrust of your statement that someone who philosophically supported slavery at a time when it was a widespread practice must necessarily be of average or below average intelligence? If so, I'd argue that you're committing two common missteps I see frequently:
1. Conflating intellectual greatness with moral goodness. These are separate categories.
2. Applying current moral standards to a previous time. You must compare him to others if his time.
Slavery is unquestionably immoral, and it's perfectly reasonable to argue that someone completing philosophical work to support slavery is committing a moral wrong, but you have to keep in mind the society into which this person was enculturated and the time at which he lived.
1. Conflating intellectual greatness with moral goodness. These are separate categories.
2. Applying current moral standards to a previous time. You must compare him to others if his time.
Slavery is unquestionably immoral, and it's perfectly reasonable to argue that someone completing philosophical work to support slavery is committing a moral wrong, but you have to keep in mind the society into which this person was enculturated and the time at which he lived.