Lesson 4 · about 50 minutes
What Is Real
How act and potency explain change, the difference between substance and accident, and the four causes that explain why things are as they are.
By the end of this lesson you will be able to
- Explain change as potency becoming act
- Distinguish substance (what a thing is) from accidents (such as colour, size and place)
- Identify the material, formal, efficient and final causes of an everyday object
Read first
- Metaphysics — The study of being as being - substance, act and potency, essence and existence.
- Cosmology — The philosophy of nature - matter, form, change and causality.
Start here: is an acorn an oak tree?
Hold an acorn in your hand. Is it an oak tree?
Obviously not: it is small, round and fits in your palm. And yet, in a way, it is an oak tree, because it has within it the real ability to become one. A pebble does not. Plant both and wait fifty years, and only one of them will give you shade.
Explaining how this is possible took some of the greatest minds in history, and their answer is the key to understanding change, identity and purpose.
The puzzle of change
An ancient Greek thinker named Parmenides argued that change is impossible. Something can only come from being or from non-being. If it comes from being, it already existed, so nothing new happened. If it comes from non-being, it came from nothing, and nothing comes from nothing. So, he concluded, change is an illusion.
It is a clever argument, but we see things change every day. Aristotle found the flaw.
Act and potency
Aristotle pointed out a middle ground between “being” and “nothing”: potency.
- Act is what something actually is right now. The acorn is actually an acorn.
- Potency is what something is really able to become. The acorn is potentially an oak.
Change is potency becoming act. The oak doesn’t come from nothing, and it wasn’t already there. It comes from the acorn’s real capacity to become an oak.
This idea is one of the most important in all of Thomistic philosophy. You will need it again in Lesson 5, when we ask whether God exists.
Substance and accident
When something changes, what stays the same?
- A substance is what a thing is: this dog, this tree, you. It exists in its own right.
- Accidents are features that exist in a substance: its colour, size, weight, place, mood.
When you get a haircut, your accidents change but your substance doesn’t. You are still you. Over your lifetime almost every cell in your body is replaced, yet you remain the same person. Your substance persists through all those changes of accident.
The four causes
Aristotle saw that to fully explain anything, we need to answer four different questions. Each answer is a kind of cause.
| Cause | Question it answers | For a wooden chair |
|---|---|---|
| Material | What is it made of? | Wood |
| Formal | What makes it this kind of thing? | Its design: seat, legs, back |
| Efficient | Who or what made it? | The carpenter |
| Final | What is it for? | To be sat on |
The final cause is the one modern thinking most often forgets. But try describing an eye without mentioning seeing, or a heart without mentioning pumping blood. Living things especially are full of purpose, and purpose is part of what makes them what they are.
Why this matters
These ideas may sound abstract, but they sit underneath almost every big question you will face. If things have natures and purposes, then it makes sense to ask what a human being is for, and what it means for a person to flourish. That question drives the second half of this course.
Read the texts
Read Aristotle, Physics II.3 and the opening chapters of Aquinas, On the Principles of Nature. As you read, ask:
- Why does Aristotle say that one thing can have several causes at once?
- How does Aquinas use matter and form to explain how something can come to be?
Primary texts
- Aristotle, Physics II.3 The four ways in which we speak of a cause.
- St Thomas Aquinas, De Principiis Naturae (On the Principles of Nature) ch.1–3 A short work Aquinas wrote for a student, explaining matter, form and change.
Check your understanding
Choose an answer for each question, then check your score.
Think and discuss
Talk these over with your class or family, or write a short answer to each.
- Every cell in your body is replaced over time. Are you still the same person? What stays the same?
- What is the final cause (purpose) of an eye? Can science fully describe an eye without mentioning seeing?
- Does everything that exists have a purpose? What about you?
- Is a human being more like a chair (made by someone for a purpose) or a tree (growing from within)?
- Why might denying that things have natures cause problems when we come to ethics?
Key takeaway
Change and purpose make sense, and things have natures.
Homework: Pick one object in your home and write down its four causes - what it is made of, what makes it the kind of thing it is, who or what made it, and what it is for.