Two kinds of purpose or teleology: Intrinsic and extrinsic

From The Imagined Conflict, chapter 5 "Philosophy of Science"

If you think of why a carpenter makes a dinner-table, Aristoteles's four explanations are: 

1. the wood which is the material cause, 
2. the idea or blueprint which is the formal cause, 
3. the carpenter who shapes the wood and this is the efficient cause. 
4. The final one is that the table is being made because someone ordered a table for a big feast. This is the final cause, which has to do with purpose or intention. 

Explanation in terms of final causes is called teleology.

In the dining table example, teleology concerns a purpose that is imprinted on the materials from outside by a human being. The wood materials themselves do not have anything in them that points toward a purpose, and if they were placed in a pile, they would never form a table by themselves. The intention comes from the person who ordered the table.This is an example of so-called Platonic teleologism, which comes from outside—is extrinsic—and is the form of teleology that is easiest to understand. In the example, the purpose comes from the person who ordered the table, but purpose can also be associated with a deity.

Aristotle did not need such an extrinsic purpose. For instance, a heart has the function of pumping blood around the body. That is the purpose of the heart, and it wouldn’t be a heart if it didn’t do that. Although Aristotle did not have the modern understanding of a heart’s function, he would probably have agreed that the purpose thus comes from within—is intrinsic—and lies in the heart’s nature, regardless of whether a god has arranged it that way or not. When you watch a nature program on television or hear a biologist speak, it doesn’t take long before such explanations appear. This is often called teleonomy instead of teleology. The term is especially used about organisms and thus denotes an intrinsic purpose. The distinction between teleonomy and teleology is meant to make it clearer that intrinsic purpose is different from extrinsic purpose.

This distinction also says something about the limitations of the machine image that is often used for an organism. ... Machines are always designed to be good for something outside of themselves, like a clock that tells the time for its user. The intrinsic purpose of organisms, we could say, is to live on, because life will reproduce itself. In a machine, each part is designed in advance to contribute to the whole. In an organism, the relationship between parts and the whole is much less clear. No part is really anything in itself, and they are not even the same all the time. The cells in the body are continuously recreated, and the cells I have today are not the same I had a few years ago—but still, I am the same person. In studies of the brain, it also turns out that it is not possible to precisely locate where things take place, unlike in a clock where each gear has a specific function. The brain has the ability to reorganize itself. The difference between machines with an extrinsic purpose and organisms with an intrinsic purpose is crucial, but still, the comparison with a machine remains a very useful, albeit limited, picture of how organisms and life function.