Setting out on a spiritual path is a little like planning a trip—to Machu Picchu, for example. Some travellers will approach the project by investing a lot of time in reading travel books or Googling Internet sites about the best route to take and where to stay—a method that works, but only to a certain extent. Other travellers prefer a much simpler and safer method: to ask someone they know and trust who has already been to Machu Picchu to go with them and show them the way. Similarly, those wishing to follow the Buddhist path to enlightenment should rely on what are called in the teachings the “four authentics”: the authentic words of the Buddha (his teachings); the authentic clarification of the teachings that can be found in the shastras (commentaries) written by great masters of the past; the further clarification that is the result of authentic personal experience; and for this experience to find expression, an authentic guru.
From Not for Happiness: A Guide to the So-Called Preliminary Practices by Dzongsar Jamyang Khyentse, page 81