The term “thinking tools” was first used by the great Greek philosopher Aristotle. He believed that slavery was a natural thing and that people were of two types-slaves and non-slaves. Some people, he said, were born slaves and should be so under all circumstances. Other people were born to manage those slaves, to be able to use them as they saw fit, and to treat them as property. Natural slaves were slaves because their souls were incomplete-they lacked certain qualities, such as the ability to think correctly.
So, they needed masters to tell them what to do. Slaves were not completely devoid of the ability to think. However, they required only a minimum amount of rational ability sufficient to understand and perform their duties. Likewise, slaves were not deprived of virtue, but they were sufficient to perform their duties again. However, this virtue was sufficient to be treated as human beings.