In the search for the highest good, he assumes that it has three characteristics that stand out despite what the highest good is , these are always the same. This must be desirable in itself. Aristotle believes that for a human being to be happy in life, he must successfully lead a life full of rational activities, because rationality is the defining human characteristic, but he also believes that it must conform to virtue in order to have a healthy soul. The Aristotelian system has the concept of "natural kinds", Aristotle found this idea attractive. It consists of species, genera and differences, these ideas separate living beings into their natural kinds, to people it seems that the animal is outside the genus. Humans are a species outside the species and the difference, what differentiates us, is our ability to reason and be rational. Because of what human beings are and our ability to be rational, we are unable to live like any other type of animal, because to be happy we must actualize and use our ability to be rational through the activity. Having divided the soul into three parts, Aristotle was then able to determine which part happiness was associated with. He came to the conclusion that because happiness is linked to the actions one performs, it must come from the rational mind.