Full Idea
For slavery to be just, according to Aristotle, two conditions must be met: it must be necessary, and it must be natural.
Gist of Idea
Aristotle thought slavery is just if it is both necessary and natural
Source
report of Aristotle (Nicomachean Ethics [c.334 BCE]) by Michael J. Sandel - Justice: What's the right thing to do? 08
Book Reference
Sandel,Michael J.: 'Justice: what's the right thing to do?' [Penguin 2010], p.201
A Reaction
Aristotle thought it met both conditions, but no one now thinks it meets either condition.