Theory of four causes:
Material cause: flour, water, and so on
Efficient cause: me, the baker
Formal cause: the idea of muffins
Final cause: to be eaten
Aristotle was one of the most important founders of the modern way of looking at things. You could say he is one of the founders of the scientific method. That is not to say that he created our modern outlook or even the scientific method. Rather, he laid the foundations which, over a couple of millennia, gradually matured into those things.
To expect him to display an understanding similar to that which a scientist displays today is ridiculous. The scientist benefits from all those centuries which have come between Aristotle and today. So you must understand Aristotle in terms of building the foundations of our current world. He did not construct the entire building.
Aristotle was trying to understand how things came into existence. Like everyone up until the time of Darwin, he knew no explanation for how things seemed to be so well created and designed for their purposes (even the very bodies of animals seem to be so) except that something must have actually designed them.
After all, when you look around and you see things that have been created or made, whether by men or by animals, they clearly do have a purpose. In these early efforts to understand the nature of reality Aristotle came to the conclusion that there were four causes for the things he saw around us that had been made.
It is important to understand that he did not believe that everything had been made for a purpose. For example, he noted that if it rained, that did the rain neither good nor bad. In other words, rain just happened. It did not need to have all four causes. For such obviously random natural events, it was enough to know that there were materials to cause the events and processes to make those materials come together to create rain or other events.
In the case of living things or objects created by living things, he felt all four causes were necessary.
First was the material cause. This was simply the materials necessary to make a thing happen. You could not build a house without wood. You could not make a statue without bronze. Even rain could not happen without water.
Second was the efficient cause. This was the individual or process which made a thing happen. Perhaps the best example is the person who builds or makes something, or the being, whether man, plant, or animal; who reproduces and creates offspring. Rain did not require a person or being to give it a clause, but there were effects of nature which would cause rain.
Third is the formal cause. This may sound a lot like Plato's forms, and it certainly is similar to that. The difference is that Plato conceived his forms is actually existing in a pure realm and in a pure state. Aristotle's idea was more like a blueprint or plan. You have the idea of the house, or the idea of your loaf of bread, and this became the form. This was a step totally unnecessary for random natural processes like rain.
This denial of a cause for rain or other natural processes later got Aristotle into some trouble. After all, almost everyone assumed that these things were caused by actions of the gods. Aristotle's denial of the gods as active agents was seen as irreligious at best.
Then there is the third area which isn't impacted by this need for a form, and that is the actions of animals or plants. Aristotle could see, as could anyone, that a spider built a web for a purpose and a goal, and yet it was also clear that the spider did not think or plan things out before it began its efforts.
There was some sort of plan, otherwise spiderwebs would never look so much the same, but the spider didn't think up the plan because the webs of each type of spider always looked exactly the same, even when different spiders made them. Nevertheless, Aristotle pointed out that since the spiders clearly did not think these things out, this was not a formal cause in the same sense as a human being's formal cause of planning a ship.
And last there is of course, the final cause. This is the reason for all the other steps being taken. In order to obtain the final cause a person gathers together the materials, the first cause. He puts his materials together, the efficient cause. He puts them together according to the plan he has made in his head, the formal cause. And he does all of these things because he wishes to attain a purpose, the final cause.
Aristotle at one point uses the teeth of animals to point out how all the causes are necessary. If someone assumes that an animal's teeth just happen to grow in a certain way because that's the way a dog's teeth just happen to grow, then how is it that they are arranged in such a way that they make it easiest for a dog to eat? Arrange them differently, and the animal might very well starve.
No one could answer that question until Charles Darwin developed his theory of natural selection. Since that didn't happen until the 19th century, Aristotle's causes have been generally accepted up until that point.
Since that time, it is more commonly accepted (among scientists and other rationalists) that there is no ultimate cause, no telos, other than those of the human mind. However, more recently we have discovered that animals, especially chimpanzees, do actually plan actions and make tools to accomplish those goals. Even more startling is the theory of evolutionary biologist Richard Dawkins as described in his book, The Selfish Gene.
Dr. Dawkins contends that the ultimate purpose of any living being is to pass on the genes contained within it through reproduction. Of course, he does not propose that the genes are actually intelligent and make this decision, but he does apply this in the same sense that Socrates applied it to a spider.
This is interesting, since scientists had prided themselves in previous centuries in moving away from the formal clause as an explanation for nature. Personally, I disagree with Dr. Dawkins. I prefer the simple Darwinian explanation of natural selection.