This is a reasonable assumption, and he illustrates his point by giving the example of medicine, which seeks health as an end. Health is the good sought by medicine.
With this point proven, he states that the highest of all achievable good is Happiness. Being human, we can vaguely understand his reasoning for this statement.
The next point he makes is that all things have a function. For this he uses the example of eyes- the function of an eye is to see. .
The next point assumption in the equation is that Man is the highest of all things, which we must accept in Aristotle's philosophy.
Putting all this together, if the highest good is Happiness, and if the highest thing is Man, we come to the conclusion that the function of Man is Happiness.
Map Of Argument.
1. All action seeks goodHappiness highest good.
2. Everything has a functionMan highest thing.
ή'nThe function of Man is Happiness.
This, of course is but the bare bones of the argument. This simple equation assumes too much, but introduces us to the concept in a direct way.
A key element to the reasoning is also explored. Aristotle seems to have written the Ethics in order to understand the nature of man. He tries to find a key difference between Humans and animals in this book also. He states that most features of humans are shared with animals, for example eyes and lungs. He answers his own question by stating that Humans are separated from other things due to the fact that we have a Soul. Aristotle believed that pleasure belonged to the Soul, that pleasure was a gift from the gods to the people on Earth (1). This makes it very reasonable to see Happiness as a good.
Now Aristotle did not mean simply Happiness' by Eudaimonia. As stated before, Happiness is only a loose translation, and human flourishing comes closer as a direct translation. Human flourishing according to Aristotle was the development and exercise of all the capabilities of the human essence (human essence being the fundamental characteristics of what it is to be human).