What we perceive intially,when we come into contact with objects, is a jumble of various inputs conveyed to us by various organs like Eyes,Ears…
This reaches the Mind as unconnected stimulii
At this level,Mind just collects information , without collating them,categorising them.
The next level of Mind,Buddhi collates the information,categorises them,tags them as correct or incorrect from a factual point of view and also passed value judgements like Good or Bad.
You would find Saguna Upasana and Nirguna Upasana,and the Union of SelfandBrahman,Eikyanusanthaana,marked.
If we look, all Yoga systems are also covered.
This was the first system by Lord Shiva and His disciple Patanjali developed a relatively easy system Asthtanga Yoga.
Shiva was the first to practice Vaasi Yoga and He is called the Adi Yogi the First Yogi.
Urdhvaredham Viroopaaksham Viswaroopayay Namaha’-Narayna Suktham.
At some point of time when Yoga is being practiced,when a practitioner is advancing,he shall have sexually stimulating visions,especially during Sakthi Upasna and he can advance only if he overcomes the stimulation and direct the flow upwards.
The process of Vaasi Yoga is Shiva.
Read Vaasi backwards,it is,Shiva.
Please check my detailed article on this.
There is an Upanishad in the,Sama Veda dedicated to Yoga process.
This was taught bt Lord Dattatreya to Sage Sankrithi.
This is the Jabali Darshana Upanishad.