"Piety requires us to honor truth above our friends," said Aristotle, who developed the idea of metaphysics without calling it that. For him, "piety" was philosophy, the ideas that rose above -- "meta" -- the known physical world. Metaphysical philosophers honor truth above all things and advocate its use in moral actions. Not surprisingly, however, history and culture have created metaphysical theories that apply different truths in different moral situations.
Aristotle: Truth is Good Actions Learned
Aristotle's theory of metaphysics applied truth directly to human actions. In other words, truth is whatever is the best moral action done for its own sake. In Aristotle's view, if I come upon my neighbor's trash cans overturned by a fierce wind, I stop and right them. Not because my neighbor will be grateful -- he may have already left -- but because I learned it's the right thing to do, without any expectation of reward. I have just contributed to the learned universal truth of well-motivated action that rises above the physical world's demands.
Descartes: Dualism and Instinctual Truth
Rene Descartes took the "meta" of metaphysics in a different direction with his ideas about dualism. Dualism holds that the mind, unlike the body, is non-material; truth is already instilled in us, and does not come from what the physical world teaches us. Aristotle's "truth" was learned. Descartes saw truth as instinctual and agreed upon. If I see my neighbor's overturned trash cans and set them right, it is because I agree with my innate need to do so. I do not necessarily contribute to a universal truth, but affirm what is true for me, for my neighbor's benefit.
Kant: Categorical Imperative and Universal Truth
The categorical imperative takes metaphysics in a different motivational direction. This philosophy, from Immanuel Kant, holds that neither Aristotle's learned truth nor Descartes' instinctive morality are entirely true. Rather, people do moral actions because a universal law commands it. This law is known to us through traditional example; it doesn't matter if we agree. If I right my neighbor's trash cans, it is because I believe, like it or not, that universal tradition demands I do so. Notice that metaphysical philosophies, despite different motivators, achieve similar ends: my neighbor's trash cans get picked up whether I follow Aristotle, Descartes or Kant.
Mill, Hume and the Greatest Good
It isn't far from the categorical imperative to the Utilitarian philosophy of John Stuart Mill, which takes universal law into majority rule: the only "truth" is the greatest good for the greatest number. I pick up my neighbor's trash cans only if I see a greater good: fewer people will be bothered, etc. I'm still picking them up, however, and I'm considering a "truth" above myself -- a metaphysical truth, in other words.
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