This disciplic succession, as Gosvāmī Hanumān-prasād said, that is essential. That is the Vedic injunction. Tad-vijñānārthaṁ sa gurum eva abhigacchet (MU 1.2.12). This word abhigacchet, it is a form of verb which is used where the sense is "You must! You must!" There is no exception. You cannot say that "I may go to a spiritual master or I may not go. I can study at home." No. You must.
Just like in modern age also, if you actually interested to be recognized as educated, you must get your admission in a recognized school or college and take degree. Then you will . . . if you study at home, you may be very great scholar, and if you say that you have passed M.A. examination, nobody will care for you. Similarly, if you actually want to be advanced in spiritual knowledge, then this is the injunction: tad-vijñānārtham.
Tad-vijñānārtham means . . . vijñāna means science. Spiritual knowledge is also a science. It is not sentiment; it is science. Now, if you like, you can make research work whether this is fact. Just like this cow dung. You may think that, "This is contradiction. In one place it is said that stool of an animal is impure, now here it is again said that cow dung is pure. It is contradiction." So if you like, you can make analysis. But you accept the Vedic injunction as it is—you save so much time, that's all, and immediately become advanced.
Therefore the Veda says that in order to learn that spiritual science, you must approach a person, tad-vijñānārthaṁ sa gurum eva (MU 1.2.12), who is guru. Guru means spiritual master. And who is spiritual master? Who abides by the disciplic succession. He does not change. He does not interpret. He presents things as they are.