A slave is a person who does not own his own labor. Therefore, the product of his labor, or some portion of it, is not his. For medieval serfs the maximum tax rate was 30%. Given the technology of the time, a higher tax rate so dispossessed serfs that the population could not reproduce, and the sefts would revolt.
19th century slaves worked with better technology, and their higher productivity meant that 50% of their work product could be taken by their owners. This was not all profit as owners had to pay handsomely for the labor, but instead of paying the laborer, the slave owner paid the slave merchant who paid the black African king of Dahomey who captured the black slaves.
In contrast, prior to the 1981 Reagan tax rate reduction, the maximum tax rate on investment income was 70% and the maximum tax rate on wage and salary income was 50 percent.
Today every Western taxpayer is a slave in the same economic sense as slaves in prior times. Today the Western citizen does not own his own labor. He owns only a part of it. The rest belongs to the slave master, that is, the government. The enslavement of the entire working population of the United States occured in 1913 with the enactment of the income tax. This enslavement is color blind.