Gururupāyaḥ गुरुरुपायः (sūtra II.6)  

Guru – the spiritual teacher; upāyaḥ - expedient. 

Only a spiritual master can lead to spiritual enlightenment. Understanding the concept of guru is very important. Guru is the one, who leads his disciple to Self-realization. The only criterion for a guru is that he should be a Self-realised person. A person with only theoretical knowledge cannot be a guru. A true guru will have the following qualities assimilated in him; self-control, austerity, purity, forbearance, uprightness, knowledge, wisdom and faith. Mere theory alone does not help in spirituality. Theoretical aspects form the foundations that are deliberated exhaustively in Upaniṣads. Spiritual progression happens in different stages. First, it begins with theoretical study and understanding them. By doing so, one acquires knowledge. The next stage is the implementation of acquired theoretical knowledge that leads to experience. From experience one derives wisdom. With the attained wisdom, one begins to explore the Brahman and at one point of time he understands that God is his own self. When this understanding transforms into affirmation, he is considered as a Self-realised person. 

The whole process is highly complicated and sometimes, theories cannot explain certain processes. If the basics are not understood properly, it leads the aspirant nowhere. He gets struck at the same level and unable to proceed further. Because of the lack of progression he becomes frustrated and decides to deviate from spiritual path, thereby losing a great opportunity to attain liberation. That is why the need of a guru is emphasised. If the guru is a Self-realised person, he will take his disciple through the correct path to realise the Ultimate Reality. It can also be said that Śaktī, through whose grace Śiva is realised, manifests in the form of a guru. This interpretation confirms two things. When Śaktī, the independent energy of Śiva, manifests in the form of a guru, it is implied that the said guru has divine grace. From another perspective, it is only through Śaktī, Śiva can be realised and She is the embodiment of all the necessary wisdom to realise Śiva. 

This aphorism highlights the importance of a right guru for true spiritual progression. Spiritual progression can be self ascertained by the level of bliss, the level of happiness without any reasoning.