It was with Aristotle that formal logic was invented, and his main contribution was categorical syllogism—a way of reasoning from premises to a conclusion. If the premises are true, the conclusion must necessarily follow. The syllogism is a form of deductive logic. We can deduce that Socrates is mortal on the basis of the premises that “All men are mortal” and “Socrates is a man.” Syllogisms can easily be programmed and, when cognitive science got started in the 1960s, many attempted to build syllogistic intelligent systems. This approach became known as rule-based or symbolic artificial intelligence.
Aristotle finds souls in everything alive—even in plants. His conception of a soul is minimalist. The soul is a set of abilities and dispositions—what makes the plant or animal behave the way it does. Souls can have four powers: those of nutrition, perception, movement, and thought. Plants have nutritive powers to nourish themselves and grow. Animals have the power of plants but can also perceive and move. Only humans have the divine power of thought—a power we share with God. How we and other animals nourish ourselves, perceive, and move are all powers explainable as bodily activities. When it comes to thinking, Aristotle suggests a special status. Some thinking is—as in Plato’s philosophy—not a bodily exercise. His account of thinking brings us closer to God, who is outside the universe and akin to a philosopher. When philosophizing, we are part of God’s eternal thinking process. Aristotle’s God is pure thinking, a being that is “thinking” and spends eternity thinking. When we philosophize, we transcend our bodies and come nearest to God—as close as we get to eternal being. The Platonic picture of reason as something above and beyond the natural world remains in Aristotle. Thought cannot be explained entirely in terms of the natural world. There is something about thinking that makes it resistant to explanations in terms of the physical world.