Before answering this question, it is necessary to explain the Buddhist doctrine of Pa?ca-yāna (Five Vehicles). Yāna means a vehicle which can carry passengers to their destinations, far or near. It is used here as a metaphor of the Buddha's teachings. The doctrine of Pa?ca-sīla (Five Pre-cepts: abstaining from taking life from sentient beings, taking things not given, sexual misconduct, false speech, the use of intoxicants) enables the practitioners to be reborn in the human world, so it is called Manussa-yāna (human-vehicle). The doctrine of Dasa-kusalānī (Ten meritorious acts—avoidance of the ten evils: 1. killing, 2. taking things not given, 3. sexual misconduct, 4. irresponsible speech, 5. speech inciting discord, 6. harsh speech, 7. licentious talk, 8. covetousness, 9. anger and 10. heterodoxy, i.e. denial of the doctrine of cause-effect) enables the practitioners to be reborn in heaven, so it is called Deva-yāna (heavenly vehicle). The doctrine of Four Noble Truths (ariya-sacca, namely, the truth of Suffering, of the Origin of suffering, of the Extinction of suffering, and of the Eight-fold Path leading to the extinction of suffering) enables the people to discard wrong views (di??hānusaya or micchā-di??hi) such as ego-illusion (atta-di??hi), eternity-belief (sassata-di??hi), annihilation-belief (uccheda-di??hi), etc., and defiled thought (micchā-sa?kappa) such as greed (lobha), hatred (dosa) and delusion (moha), finally, to attain nibbāna(enlightenment). So it is called sāvaka-yāna (voice-hearer vehicle). Voice means the Buddha's teachings. Those who hear the Buddha's teachings, realize the Four Noble Truths and are free from sufferings are called sāvakā. Before the advent of the Buddha-Dhamma, people who independently came to the realization of the Theory of Pa?iccasamupāda (Dependent Origination) and obtained emancipation, but were unable to speak out their self-enlightened truth were called Pacceka-Buddha (Independently Enlightened One). That is why the Dhamma of Dependent Origination is called the vehicle of self-enlightenment (pacceka-yāna). The doctrine of Six Perfections (cha pāramitā) enables the practitioners to act in the way of Bodhisatta, to undergo the countless difficulties and hardships in cycles of birth and death, and to finally attain Buddhahood. So this teaching is called the vehicle of Bodhisatta (Bodhisatta-yāna).
要說明這個問題,先要談一談佛教的五乘教法。乘就是車子,能運載行人到遠近不同的目的地,借以比喻教法。五戒的教法(戒殺、盜、淫、妄、酒的五條戒)能令修持者得生人間,叫做人乘。十善的教法(不犯十惡即是十善。十惡是指:一、殺;二、盜;三、淫;四、妄語;五、兩舌,即挑撥離間;六、惡口,即粗惡傷人的言語;七、綺語,即邪淫言語;八、貪;九、嗔;十、邪見,即否認因果的見解。)能令修持者得生天界,叫做天乘。四諦的教法,能令人斷除見惑(我見、常見、斷見等錯誤見解)和思惑(對世間事物而起的貪嗔癡等迷情),證得涅槃,叫做聲聞乘?!奥暋笔茄越痰囊馑?,聽聞佛的言教,悟四諦之理而得到解脫的人叫做聲聞。在沒有佛法的時代,有人獨自悟到緣起之理而得到解脫,但他不能把自己悟到的真理說出來,這種人叫做獨覺,因此十二因緣法叫做獨覺乘。六度的教法,能令修持者行菩薩道,經(jīng)過無數(shù)世的難行苦行,最后達到佛的果位,叫做菩薩乘。