There are said to be three principle aspects of the path to Buddhahood -- renunciation, bodhicitta and wisdom. These are called "principle aspects" because of the crucial role they play in following the path. Most of the other aspects of the path can be understood as either steps necessary to attain renunciation, bodhicitta or wisdom, or as expressions of these three. In order to become a Buddha and achieve true and lasting happiness, one must cultivate renunciation, bodhicitta and wisdom. The practice of cultivating the belief that every living being has been one of your mothers, for example, is an optional element of the path, because it is part of a method, not the only method, of cultivating bodhicitta. So, one would not say that the belief that every living being has been your mother is a principle aspect of the path to Buddhahood.
Bodhicitta is a principle aspect of the path, because it is the aspiration to become a Buddha for the benefit of all. Obviously, if we do not have this aspiration, we are not going to become Buddhas; you cannot accidentally become a Buddha. The five perfections of effort, generosity, patience, moral discipline and concentration (as well as wisdom) are ways of expressing bodhicitta, because they are steps on the path to fulfilling our aspiration and because each virtuous act is chosen with bodhicitta motivation. It is not correct to say that bodhicitta is an expression of patience, but patience is an expression of bodhicitta. For this reason, bodhicitta is called a principle aspect of the path, but the first five perfections are not. By way of analogy, if we want to travel to Chicago, we would call this our goal, and we would say that we are traveling down a particular highway because we want to reach our goal. In this way, the goal is more central to what we are doing than the steps we need to get to our goal -- the steps are chosen because of the goal, not vice versa.
Buddhahood is a state of mind; it is the mental activity of directly perceiving the way things truly exist, and it is the mental activity of always acting with the intention to benefit every living being. Buddhahood is also a capability -- that of being maximally endowed with the ability to help all sentient beings. All of these elements of Buddhahood come from wisdom; a wise person always has these states of mind and this ability. For this reason, wisdom is a principle aspect of the path.
Renunciation may be the most misunderstood aspect of the path, because many people assume that Buddhists must totally withdraw from worldly activities and live the life of an isolated monk or nun in order to reach enlightenment. To people who believe this, renunciation is an extreme form of asceticism, but this is false. Buddhism is called "the middle way," partly because it is the middle path between the extremes of trying to find happiness through indulgence in mundane, worldly activities on the one hand and, on the other hand, trying to find happiness by practicing extreme asceticism. Renunciation is actually an attitude about the value of mundane, worldly activities and things, not a total withdrawal from "normal" life activities. It is not necessary to abandon most of one's material possessions in order either to become a Buddha or function as a Buddha. But Buddhas don't need much of anything in order to be of great benefit to others. Shakyamuni Buddha's teachings continue to be of immense benefit to all, but he possessed only the clothing he wore and the alms bowl he carried when he gave these teachings.
Renunciation is the conviction that true and lasting happiness cannot be found in the possession of material wealth, power or fame. These mundane things can, at best, only be instrumentally useful when skillfully used in the spiritual practices that do bring true and lasting happiness. A Buddha can possess wealth, power and fame, because they can be useful tools for helping others, but Buddhas find happiness in the mental states they have achieved, not from the things they possess. Achieving renunciation is the first crucial step on the path to Buddhahood, because as long as you believe that happiness can be found by possessing mundane things, you are not going to engage in the spiritual practices that do lead to true and lasting happiness. You, also, will not engage in effective and authentic spiritual practices as long as you think that happiness is found in a place of abode called "heaven." Jesus and Buddha both taught this. Jesus communicated this when he said, "the kingdom of God is within you." (Luke 17:21) Clearly, what Jesus meant by "within" is within your heart and mind. In order to achieve true and lasting happiness, you must train your mind until you reach Buddhahood or, in other words, find the kingdom of God that is within you.
In summary, renunciation, Bodhicitta and wisdom are the three principle aspects of the path, because these accomplishments, jointly, are the necessary and sufficient causes of true and lasting happiness. One must first develop the attitude of renunciation in order to begin to abandon dysfunctional, mundane goals; next, one must develop both aspiring and engaging bodhicitta in order to travel the authentic spiritual path; and, finally, one must acquire the wisdom that is the state of mind of someone experiencing the true and lasting happiness of a Buddha.
For more by Tenzin Norbu, click here.
For more on spiritual development, click here.