To understand the nature of the mind, there are two ways: one is easier, the other is harder. The easier way involves your lama introducing you to the nature of the mind, and on that basis you work on that and come to know the mind and its nature. As for the harder way, even if a lama introduces the nature of the mind, still it may be difficult to understand its nature. Therefore, to really understand the true nature of the mind, the introduction by the lama must be supplemented by faith, belief, and longing supplications. Then you will receive the siddhi of your guru. Many people can talk about the nature of the mind, but when you have a karmic connection with a lama, even though the words are the same, the effect is different.
His Holiness the 17th Karmapa