[Kliopeion]

On a somewhat different note (hence the separate post), my understanding is that the bodhisattva aspiration is interconnected with the cultivation of prajnaparamita. It's the attainment of prajna -- the insight into emptiness -- which makes the vows realizable. Otherwise, as Aloka mentioned above, they appear to be unrealistic or even impossible.

Red Pine, in his book on the Heart Sutra, discusses this at some length.

The liberation of all beings revolves around the liberation of bodhisattvas from the concept of being. Only when bodhisattvas find no beings to liberate are they ready to complete the bodhisattva's path to buddhahood. And later:

The only qualification to walk (the bodhisattva path) is the twofold vow to attain enlightenment and liberate all beings. Of course, no one would or could make such a vow if they did not practice the Prajnaparamita. For without the Prajnaparamita, the vow is simply too overwhelming to attempt, much less fulfill. But once it is seen in the light of the Prajnaparamita, this vow is open to all, regardless of their capability or preferred form of practice. This is because the limiting categories of time and space disappear. Though it takes countless aeons to liberate all beings, the bodhisattva asks 'What aeons? What beings? What liberation?"