Yogis believed that there is a connected spiritual oneness in all existence. This oneness or unity is clearly emphasised in the name of yogi's spiritual journey; derived from the Sanskrit root ‘Yuj’, Yoga means to yoke or unite. 

Over millennia many schools of thought and branches of yoga have developed and proposed different practices and tools on this path, and often with conflicting views, but the goal of each still remains the same - Yoga is and always has been a path of Self-realization leading to the ultimate goal - liberation (Moksha). Through different practices and wisdom of Higher Self, a yogi is trying to free oneself of the Ego-bound perceptions or illusions of the material world and realize the divine within. This divine, according to Katha Upanishad, "resides in the middle of the body" - the seat of our heart.

Nondualist and dualist schools of thought

The Advaita, or nondualist, school of thought sees the individual Consciousness or Soul (Atman), as unseparated from the Supreme Consciousness (Brahman). In fact, everything around us is only a different form of Brahman, as Brahman is everywhere and inside each living being, the binding unity behind all that exists in the Universe. The realization of this Truth leads to liberation or freedom. Advaita Vedanta, Jnana yoga lies upon, as well as most branches of Tantra, from which Hatha yoga emerged are essentially nondualist branches of yoga.

The dvaita or dualist school of thought recognises two independent principles; Purusha (Soul as well as the Supreme Soul) and Prakriti (nature of matter) and teaches that they are two distinct aspects of reality. We see this clear distinction in Samkhya philosophy, as well as Patanjali's Yoga Sutras. However, the path of yoga still teaches the yogi(ni) to free oneself from the attachment to the material world and see his/her divine nature. 

The differences in how these two philosophies saw the world are not really important, what is important is that spiritual seeker continues to see there is a divinity within and without. Every spiritual path is the light to that Truth.

“If there is one common doctrine that runs through all our apparently fighting and contrary sects, it is that all power, all glory and purity are within the soul already; only according to Ramanuja the soul contracts and expands at times and according to Shankara it comes under a delusion. Never mind these differences. All admit the truth that the power is there, potential or manifest, it is there and the sooner you believe that, the better for you.” - Vivekananda

And, what about asana?

Yoga and different yogic practices have existed for thousands of years before asana. Asana, or a very small part of physical postures we practice today, have emerged in Hatha yoga, within the Tantric era, a few millennials after Yama (the God of Death) explained to a boy called Nachiketas what Yoga is:

When the five organs of perception become still, together
with the mind, and the intellect ceases to be active:
that is called the highest state.

This firm holding back of the senses is what is known as Yoga.
Then one should become watchful, for Yoga comes and goes.

(Katha Upanishad 5th century BCE translation by Swami Paramananda)

Centuries later, in his Yoga Sutras, Patanjali states that "Yoga is the stilling of the changing states of the mind".  In his eight-limb path to Samadhi (the highest state of Self-realisation) for which he first lays the moral foundations of a yogi (Yamas and Niyamas) he proposes similar ideas of how to reach the state of Yoga to that found in Katha Upanishad - finding a confortable seat (Asana) a yogi should practice Pranayama, Pratyahara (holding back of the senses), Dharana and Dhyana (concentration and meditation) to still the fluctuations of the mind and reach the state of Yoga and ultimately, Samadhi.

Much later on, during Tantric era, physical body was recognised as an important part of the whole. Thus, Hatha yogis developed different techniques to keep the body healthy, and asana was practiced to keep the body sheath (annamaya kosha) strong and healthy, as well as to learn to control the body. Because, if you can’t control the gross - your body, food you consume etc. and can’t keep it sattvic (pure), how can you control the subtle (mind)? 

Asana is a great tool of yoga, but in comparison with others, only a small piece of the puzzle. If asana becomes a goal, and in order to reach that goal you cause harm to the physical and/or subtle (injuries, strains, self-doubt, competition, feeling of failure etc), something we see so much of today, it causes harm, feeds the Ego, and in some cases, narcissistic tendencies. If the ultimate goal of yoga is to see the divine within and without, the obsession with asana and the physical reality is, instead of closer, taking us further from the Truth. 

But, I practice non-spiritual yoga

Non-spiritual yoga? 

Without being judgmental, only truthful, I see this as an absurd notion. It's like saying you practice a non-spiritual Buddhism, a non-scientific physics, or a non-philosophical metaphysics. Just as you can't separate Buddhism from its spiritual roots, you cannot separate Yoga from its spiritual essence. 

If you are confused with all the above concepts, it's normal. Yoga today has become so much about asana, alignment, how to and how not to, the anatomy and biomechanics of it all etc., you kind of have a feeling it was never anything more than the physical practice, and this couldn't be further than the truth. Another issue is, today many things are named, often misleadingly, using the term yoga. I believe this isn’t right or truthful. If you teach and/or practice asana, meditation and some breathing, but don’t believe in the spiritual aspects of Yoga, you are using yogic tools to enhance the health and wellbeing, which is beautiful and smart, but it doesn’t make you a yogi. Same as wearing a Buddhist mala, Buddha statues all over my apartment and being curios about Buddhism doesn’t make me Buddhist.

I can, however, see why this is happening. What we call Yoga has become a multi-billion business and calling something asana practice or someone an asana teacher just doesn't have the same allure. Just like sex sells, yoga sells, and this, to me, is heart-breaking and counterproductive to all of us. What world needs right now is more spirit and less materialism, more union and less separation, more Truth and less pretence.