The History of Life: Aristotle’s explanations


Aristotle is (in)famous for his four causes, although they are better thought of as types of explanations. They are usually given standard names, but to make them a bit clearer, I’ll give some simpler  names first (just as Aristotle did in Greek).

  • Composed-of (the material cause): the properties something has due to the material it is made of;
  • Form-of (the formal cause): the properties something has in virtue of its shape;
  • Moved-by (the efficient cause): the properties it has from some external force;
  • Purpose-of: (the final cause): The reason something exists for.

Now the last one, the final cause, is a bit odd to modern eyes. It is not a cause in the sense we understand it, because unless causes can operate from future to past, the purpose of something is not what makes it be what it is, unless prior to the thing existing someone intended it to be for the purpose. An example might help confused modern readers.

Supposed we have two wheels made out of different materials, brass and wood. The weight and strength of the two wheels clearly is due to the stuff they are made of. That is a material cause. They both roll because of their shape. That is a formal cause. What makes the wheels roll rather than stand still is the efficient cause (in Aristotle’s physics, things do not move if they are in their natural place, and so they need something to move them). And the craftsman who made the wheels had a purpose for making them, which is the final cause.

To our eyes this is strange. The key confusing term here is “cause”, which in Greek is aition. It comes from a root word that means “fault” or “culpability”. Aristotle (and his predecessors) ask what is the culpability of some property. It has been suggested that the better English equivalent is “account”. In this interpretation, his “causes” are logical accounts of the properties of the object (the technical term for logic in Aristotle is “syllogistic”).

This puts a different spin on Aristotle’s notion of causation. Since the final cause in particular plays a large role in biology (the “function” or purpose of living parts, for example), it is important to note this. Later, with the rise of early modern science in the 16th century, final causes became contentious, although they remained in biology for much longer, being called teleology.

Form and Substance

Aristotle also has a physics that plays on the first two accounts, the material and the formal. He opposed atomism and the existence of a void* between them. All space must be filled by a substance, and upon that substance a form is imposed so that it has the properties it has. This view, the substance/form division, is called hylomorphism. It was the ruling physics of the west until the development of modern atomism by John Dalton in the 1800s. Aristotle uses this notion to explain much of biology, including the mind.

In Aristotle’s metaphysics, form (morphē) explains the external properties of things like organisms. It is imposed upon the substance and does not, unlike Plato’s Forms, exist independently of them. The “soul” (psuchē) is a form that moves organisms, and the mind’s form (nous) is the soul that permits reasoning. Aristotle has form and matter (substance in Latin) as a single unity of being (ousia). The shape of wax is not separable from the substance wax, for example. When a thing changes, what changes is its form.

Aristotle soul functions

This leads to Aristotle’s tripartite distinction of souls in De Anima (On the Soul, follow link for a short discussion). There are three things that living bodies do which is proper to them:

  • Grow by way of taking in nutrition
  • Move and perceive things
  • Think or have intellect.

This gives living things three possible souls:

  • The Nutritive soul (all living things have this; plants have only this)
  • The Sensitive or Locomotive soul (all animals have this); sometimes the “Appetitive soul” is added in here or treated as separate.
  • The Rational soul (only humans have this).
This led to a long-lasting (even today) distinction between “lower” forms of life and “higher” forms of life that came to be known as the Great Chain of Being. We shall cover this much later. As reconstructed by later writers, Aristotle’s view can be diagrammed like this:
Ladder of Life
From Charles Singer, A Short History of Biology.

One interesting thing about this is that it does not form a single linear arrangement of life, in contrast to later forms of the Great Chain. There are numerous things that exist in the nutritive “layer”. The other thing to note is that this is not divided sharply into genera or classes. It is a continuum, and the least complex things arise out of matter in what came to be known as spontaneous generation, which we shall encounter again later.


∗ The phrase “nature abhors a vacuum” is much later, due to Rabelais in the 1530s.

† In Aristotle’s scheme, something can be proper or accidental. A proper feature is one that cannot be removed without the thing ceasing to be that kind of thing. An accidental feature can be removed and the thing remain that kind of thing. So a bird properly has a beak or bill, but accidentally has the colour white.

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