One of Aristotle's four causes is the formal cause. The SEP writes:

The word “form” may misleadingly suggest that what is acquired in a case of substantial generation is simply a shape, and this impression is reinforced by some of the examples that Aristotle uses, especially when focusing on artefacts: plausibly the form of a bronze statue just is its shape. When we consider organisms, however, it becomes apparent that having the right shape is not sufficient to possess the form. A thing’s form is its definition or essence—what it is to be a human being, for example. A statue may be human-shaped, but it is not a human, because it cannot perform the functions characteristic of humans: thinking, perceiving, moving, desiring, eating and growing, etc. The connection between a thing’s form and its function emerges in Physics ii 3, where Aristotle distinguishes his four kinds of cause: material, formal, efficient, and final, and suggests a special connection between the formal and final cause.

But how can a reference to the form of an object actually explain anything? It is a banality that the organizational structure of an object is one of the main sources to explain the functioning of that object, but that's just where the real work begins, which is about efficient causes.

Could you give me some examples, where the formal cause has real explanatory power?

  • See Aristotle : The Four Causes in the Science of Nature : "the form and the end often coincide, and they are formally the same as that which produces the change (Phys. 198 a 23–26)." Dec 7, 2016 at 12:56
  • A's universe is theleological : (Phys. 198 a 36 "the essence of a thing, i.e. the form; for this is the end or that for the sake of which. Hence since nature is for the sake of something, we must know this cause also." Dec 7, 2016 at 13:01
  • A reasonable approximation from a modern perspective might be that formal cause is constituted in whatever underlies language and gathers thoughts into semantics. The thought 'I love my dog' is a formal cause of the relationship you maintain with the animal. Absent the thought, the relationship ceases to exist. But the thought is not a purpose, a material thing, or a constitutional process. It is pretty much capturable only mentally. Likewise the shape of a statue is a complex mental impression like a description, and the fact you are a person is equally a semantic artifact.
    – user9166
    Dec 7, 2016 at 18:16
  • Aristotle did not believe that efficient causes explain the form of objects produced (this came out of mechanicism of 18th century). Not only in the cases of artistic creation of a statue, where it is obvious, but even in the case of a plant growing into its mature form. Material causes explain the materials for, and efficient ones explain the mechanics ("mover") of the shaping, but neither explains the shape. Aristotle's formal and final causes often coincide, “a thing’s form, or what it is, for that is its end and what it is forfaculty.washington.edu/smcohen/320/4causes.htm
    – Conifold
    Dec 7, 2016 at 21:51
  • @Conifold: what do formal causes explain then?
    – viuser
    Dec 7, 2016 at 22:43

3 Answers 3


One point is that the efficient cause, on which you concentrate, needs something to operate on, something to apply to. The form determines or limits what the efficient cause can produce. How in any case could the efficient cause operate on something formless ? But this is the short answer and we need to take a longer road.


At its deepest level, Aristotle tells us in Metaphysics, A, 1, 981b28, philosophy looks for 'the first causes and the principles of things' (J. Barnes, 'The Complete Works of Aristotle', 2, Princeton : Princeton University Press, 1984, p.1553). To know these causes and principles is to have wisdom (sophia).

Metaphysics, Δ, 1, 1013a17 (Barnes, 1599) identifies causes with principles, so we can just talk about 'causes' (aitiai) from now on. The divergence between aitiai and causes will emerge as we go on.


In Metaphysics Δ, 2, 1013a-1014a and Physics II, 194b-195b, Aristotle distinguishes four causes : the material cause (to hupokeimenon), the formal cause (to to me einai), the efficient cause (arche tes metabolis) or cause of change, and the final cause (to telos, ou heneka) or that for the sake of which.


'Matter' (hule) for Aristotle is that out of which a thing becomes. So while 'matter' may be the bronze from which a statue is made, it may also be the seed from which a plant grows, the premises of a conclusion, the letters from which a word is formed. The hallmark of matter is that it is something potential, the dunamis or possibility of being formed into something. It is always incomplete or what is not yet - something passive which requires determination into something specific.


This determination, or fixing of specificity, is the formal cause. Aristotle uses a variety of phrases for this : to to me einai (as above), morphoi, logos, eidos, paradeigma. Form relates to matter as that which determines the passive into something determinate (or more determinate). Thus the shape of a statue which the bronze is transformed into, the word into which the letters are arranged, the whole which combines elements into parts, the law which regulates the behaviour of objects, the specific difference which distinguishes a species within a genus : all these are formal causes. The formal cause is what makes the determinable determinate.


The formal cause of X is the essence or essential nature of X, its essential property where a property belongs essentially to X only if X would cease to exist without that property. The identification of formal cause with essence is clear from an abundance of texts : Metaphysics Z.7, 1032b2, Phyics 2, 194a21, 3, 194b26, de Anima 1, 412a20. An essential property, hence a final cause, is also explanatory.

There is also a link with definition. When one has identified the formal cause of something and pinpointed its essential properties, one also can give its definition.


One obvious application is to the question of identity. Someone, Y, a human being, can change in many ways over time. S/he can grow, lose their job, change occupation, contract a disease, become the citizen of a different state, be reduced to poverty, have a limb amputated but still remain a human being and the same human being. But if s/he ceases to be a living organism, this is an essential property and losing it means ceasing to exist as a human being.

Being a living organism also fulfils an explanatory role in relation to other properties that Y has. If Y is a human being, then ceasing to be a living organism entails losing all the properties that belong to Y as a member of the species, human being.


If we follow the exposition above, it is clear that whatever difficulties attend the concept of a formal cause, triviality is not among them.


Formal causality explains what (quid) something is. Efficient causality explains how something comes about.

Intrinsic causes:

  • Material: answers "Out of what (ex quo) is it?"
  • Formal: answers "What (quid) is it?"

Extrinsic causes:

  • Final: answers "Why or wherefore is it?"
  • Efficient: answers "Whereby is it?"

Adapted from this answer to the question "Is conception an Aristotelian efficient, or material cause?"

Aristotle discovered the four causes in order to explain that motion is possible (contra Parmenides, who denied the possibility of change) and that, despite there being a plurality of beings in the universe, there are stable beings (contra Heraclitus, who thought everything is in constant flux).

Since the real distinction between actuality and potentiality is so important to Thomism, a good overview of Aristotle's solution to the Parmenides vs. Heraclitus controversy on change is given in Part II, "The doctrine of actuality and potentiality and its applications according to St. Thomas," of the free book The Essence & Topicality of Thomism.

Another good short work on this topic is Thomas Aquinas's On the Principles of Nature (De principiis naturæ).

From here, the first part of this work overviews the Aristotelian philosophy of nature:


The first half of Jack Sander’s Philosophy of Science Lecture #8: Scientific Explanation provides a clear explanation of Aristotle’s model of explanation emphasizing that it is still of value today.

The formal cause tells us what class something belongs to. The desk I am writing on has the formal cause of being a desk. Of all the things that it is like, my desk fits into the class of desks. Once I know that it is a desk all sorts of possibilities open up as to how I might use it because I know what desks are. The computer I am using fits into the class of computers. Once I know it is a computer and not, say, a spaceship for very tiny aliens, I understand how I might relate to it. Suppose someone tells me there's an animal outside. If I am told it is a lion I will respond differently to it than if I was told it is a cat.

Given these examples it might be interesting to ask if formal causes can be reduced to some combination of the other three causes in Aristotle’s model of explanation. That is basically asking whether formal causes have “real explanatory power”. If I don't need to directly ask what something is to find out what it is, the formal cause, "What is it?", is not as valuable in the model of explanation.

Suppose someone has an object hidden in a box and I am supposed to guess what it is. I am only allowed to ask three questions, one for each of Aristotle’s causes, except I am not allowed to ask, “What is it?” I have to find out the formal cause from information about the other three causes. Furthermore I will only get a partial answer to my questions because this someone holding the box doesn’t really know everything there is to know about what is in the box and I would not have time to listen to a complete explanation if that person did.

If I ask, “What is the mystery item made out of?”, this would be asking for the material cause. The person might say it is made out of plastic, wires and metal. That’s interesting, but I still don’t know what it is and the goal is to find out what it is without asking what it is.

If I ask, “How was it made?”, this would be asking for the efficient cause. The person might say it was made by some electronics manufacturer. That narrows my guesses down about what the thing is in the box, but I still don’t know.

Finally I ask for the final cause, “What is it for?” I am told it is for communicating. I still don’t know for sure. I might guess a phone, a computer, a laptop, a monitor, but once I find out it is a modem, that is, once I know the formal cause or what it is, I know enough about what is in that box.

Although the other three causes may help me guess what something is, knowing what it is provides real explanatory power.

  • Nice answer. I particularly like the second para. Another Hubeny contribution that carries heft !
    – Geoffrey Thomas
    Mar 19, 2018 at 8:23
  • @GeoffreyThomas Thank you! I like your answer also. You bring an impressive depth of experience to the questions you have been answering. Mar 19, 2018 at 13:06

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