Were it truly self-evident, it would have been self-evident to the Romans, west Africans, Caribbean English, and all the other slaveholders in history. What is self-evident to us is that it clashes with our particular moral system.

Strictly speaking, a slave is not utterly robbed of agency – but rather has his or her agency constrained to what are usually unpleasant options. The Barbados cane cutter could work, run away, attempt to kill his oppressors, do nothing and endure the beating, or commit suicide. All bad choices, but still an expression of individual agency. Only a condition like full physical paralysis would leave a person with no agency at all. In a technical sense, this causes trouble for your definition because we are all constrained by all sorts of conditions, so questions of agency become murky rather quickly. Another problem with the agency argument is that, at least hypothetically, a slave might have desires that generally lined up with those of his owners. If a slave is content, but still the legal property of another, is he not a slave nevertheless? This is why I chose a definition that defines slavery in terms of ownership rather than agency.