Tuesday, July 19, 2011

Aquinas: Metaphysics

Act and Potency

Things in the world have ways they actually are, and potentials that can be actualized. A rubber ball is actually red and round, but is potentially a puddle of goo.

  • Potentialities only consist of things rooted in a thing’s nature; not just any potentiality we can conceive of, like the rubber ball spouting teeth and going around eating things.
  • Potentiality cannot become actual without being changed by something external that is actual, since the potentiality doesn’t exist yet. Also, if it could actualize itself, it would have done so already.
  • Nothing that exists can be just potential. Most things will be a combination of act and potency, and some, such as God, are pure act.


The ordinary objects of our existence are composed of matter and form, which makes them also compounds of act and potency. Form is prior to, or more fundamental than, matter, paralleling act being more fundamental than potency.

  • A thing has substantial form, which makes it what it is (a ball must be spherical and bouncy), and accidental form, which is not essential to what the thing is (the color of the ball).
  • Just matter by itself without any form is called “prime matter”, and cannot exist as it is purely potential (see above). However, like pure act, pure form can exist on its own.

Four Causes

What a thing is made of is its material cause, what form that material takes is its formal cause, what brings it into existence is its efficient cause, and what effects it produces (if any) is its final cause.

  • The material and formal causes go hand in hand (see hylomorphism, above).
  • Efficient cause and final cause are necessarily connected as well, in the same way. If A is the efficient cause of B, B is the final cause of A.
See Summa Theologica, Book I, Question 44, Article 4, “I answer that...”

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