You might want to look at this and this before continuing.

The final cause is the fourth of the four causes, and it is in ill-repute this days; but I hope to see it rehabilitated. Why is it in ill-repute? Aha! There-in lies the tale.

Consider a rock perched on a cliff. There is a slight earth tremor, and the rock falls, landing at the bottom of the cliff. The material cause is the rock. The efficient cause, or agent, is the tremor. The formal cause is the rock’s new position. The final cause is…what again? The end of the change is simply the new position; there doesn’t seem to be anything special about it.

Consider a green apple that turns red. The material cause is the apple. The efficient cause is also the apple; it is its nature to change color. The formal cause is the form of redness. And the final cause, ah! The final cause is the ripened apple, or perhaps, simply “ripeness”. Aristotle called the final cause “that for the sake of which”; and surely the apple turned red for the sake of being ripe.

Consider the apple tree, which sprouts leaves. The material cause is the tree, or perhaps the nutrients the tree pulls from the soil and from which it makes the leaves. The efficient cause is the tree; sprouting leaves is part of its nature. The formal cause is a leafiness; and the final cause is nourishment, because the leaves turn sunlight into food for the tree.

With living things, we begin to have a notion of purpose: the living thing does what it does to achieve its purpose of remaining alive. Leaves are for producing nourishment. Hearts are for pumping blood.

Consider a man who makes himself a peanut butter sandwich. The material cause is the bread and the peanut butter. The efficient cause is the man himself. The formal cause is the form of a sandwich. And the final cause is eating. The sandwich is made for a purpose. It is a means to an end. The sandwich is made for eating, and eating is made for nourishment. With human action we see not only the objective purpose of a nature, but the freely willed purpose of a person. With human action we begin to see chains of means all leading to some end.

So what is the final cause of that rock falling to the ground? I’ve often heard it said that in a case like this, the final cause is simply the formal cause: the ending position of the rock, perhaps described as a point of minimum gravitational potential. There’s no purpose to the ending position of the rock, not like there is for ripeness or for leaves or for the peanut butter sandwich.

And yet there is—for the rock is part of creation, and creation was created to glorify God. Merely by being a rock, in all the simplicity of its nature, the rock joins in creation’s choir of praise. Merely by falling by force of gravity, it is operating according to its nature and glorifying God. It is a means to that end.

Here, of course, we’ve gone beyond philosophy and into theology; and we’ve seen several reasons why final causes are in ill-repute.

First, because in non-living systems you don’t seem to need final causality to explain them. And the paradigm of science is based on physics, which is all about non-living systems. (But just because physics isn’t interested in final causality doesn’t make it false.)

Second, because final causality is seen as being about purpose, and especially the purpose of a person; and this is seen as leading to theism, which is also in ill-repute in some circles. But I’ll note that in biological systems, i.e., organisms, we clearly see purpose at work. The apple ripens for its own purposes. The heart is clearly for pumping blood. Not that either one is a moral agent on its own. If you’ve heard people talking about teleology, this is ultimately what they are talking about. But the same people who dislike final causes also regard teleology as the statement that God designed everything himself (in the Intelligent Design sense); but philosophically the notion of final causes is much more modest. (Acceptance of final causes does lead, ultimately, to theism; but it doesn’t presume theism on the face of it.)

There’s a third reason, too. The final cause, or end, doesn’t exist when the change begins. How can we call something a cause of a change, when it doesn’t yet exist? Interesting question.