“Vehicle” (yana) has two meanings: the means by which one progresses and the destination to which one is progressing. Mahayana in the sense of the vehicle by which one progresses means to be motivated by the mind of enlightenment—wishing to attain highest enlightenment for the sake of all sentient beings, one’s objects of intent—and means to engage in the six perfections.
Seeing reason and need, Buddha set forth many systems and vehicles, but these did not arise due to his being intimate with some and alien to others. The trainees who were listening to his teaching had various dispositions, interests, and abilities, and thus he taught methods that were suitable for each of them. For those who temporarily did not have the courage to strive for Buddhahood or who did not at all have the capacity of obtaining Buddhahood at that time, Buddha did not say, “You can attain Buddhahood.” Rather, he set forth a path appropriate to the trainees’ abilities. Buddha spoke in terms of their situation, and everything that he spoke was a means of eventually attaining highest enlightenment even though he did not always say that these were means for attaining Buddhahood.
Since the purpose of a Buddha’s coming is others’ realization of the wisdom of Buddhahood, the methods for actualizing this wisdom are one vehicle, not two. A Buddha does not lead beings by a vehicle that does not proceed to Buddhahood; he establishes beings in his own level. A variety of vehicles are set forth in accordance with temporary needs.
From Tantra in Tibet, pages 43–44