Most of us take up the practice of yoga knowing only that it is good for us. That's precisely what I did. I liked the idea of stretching and relaxing; of getting into better physical shape. Maybe chill out a bit too and get a little wiser. It just seemed a neat way to make improvements to my life for very little time or money. Then I discovered there were styles of Yoga. Different teachers emphasised different principles and organised them in different ways. Bodily alignment was key to better posture in everyday life. Place your foot in this or that position and amazingly the hips open; the spine comes into better alignment. Progress was easy to measure: become expert in this posture and that one – the greater the number of postures mastered, the greater the teacher/student; lengthen inhalations and exhalations and so on.
What never occurred to me in the first few years of my yoga practice was to ask the question: What is Yoga?
It might seem an odd question, but actually understanding the basic rationale for anything deepens one’s appreciation of it and informs one’s judgement about its usefulness and relevance. It helps one to know where yoga could take you. Understanding the answer to this question is vital because it tells us something about the direction in which the practice of Yoga is likely to take us. This applies to pretty much anything. Making cakes and eating them can lead you to opening and running your own patisserie, hosting a TV show or putting on a lot of weight or all three!
It turns out that there is no single answer to the question: What is Yoga?
This can’t come as a big surprise really when one considers how old Yoga might be. It’s old: more than 2,500 years old. It appears to have developed and evolved to meet the needs of each new generation of adherents in different religious, cultural and Geo-political environments.
So how do some of the surviving ancient Indian texts define Yoga?
The Kaṭha Upaniṣad says that the state of Yoga arises when the senses are firmly under control . The Bhagavad Gītā gives several definitions including these three important ones: Yoga is the perfect evenness/steadiness of mind (samatvam); Yoga is skilful action (karmasu kauśalam); and Yoga is separation from suffering and pain (duḥkha-saṁyoga-viyogam) . The Yoga Yājñavalkya says that yoga is the union of the ‘individual’ (jīvātmā) and the ‘divine’ (parāmatmā) .
The Yoga Sūtras offers this definition: ‘Yoga is the ability to direct the mind exclusively toward an object and sustain that direction without any distractions’ and in so doing achieve the separation of the ‘individual’ from the ‘material world’. [4 & 5]
Making sense of these seemingly disparate modes of understanding of yoga's ultimate goal is tricky. TKV Desikachar skilfully rephrased these ideas for our modern western sensibilities:
All definitions of Yoga ‘have one thing in common: the idea that something changes’
‘One of the basic reasons many people take up yoga is to change something about themselves: to be able to think more clearly, to feel better, and to be able to act better today than they did yesterday in all areas of life’
Yoga can mean: ‘to attain what was previously unattainable. The starting point for this thought is that there is something that we are today unable to do; when we find the means for bringing that desire into action, that step is yoga’
‘Yoga attempts to create a state in which we are always present … in every action, in every moment’
‘The practice of yoga only requires us to act and to be attentive to our actions’ 
Nowhere is yoga being defined as a system of physical exercise. Rather it is being defined in terms of the mind, how to focus and direct it, the quality of actions that spring from the attentive mind and the movement away from suffering. This was a bit of a shock to me; still is, if I am honest.
We can measure and test our progress in practising shoulder stand for 10 minutes every day, knowing how to prepare for and finish a practice focused on shoulder stand, breathing in certain ways and incorporating the appropriate bandhas. One could judge these qualities in a competitive arena in much the same way that gymnasts or dancers are.
However, measuring the efficacy of yoga, as defined by great yogis in the ancient texts that mention yoga, requires us to think differently.
Not long ago, I was browsing in a second-hand bookshop and picked up a book on Western Philosophy. I was struck by a quotation on the back cover, which was attributed to Epicurus, the 4th century BC Greek philosopher, and how yoga-like it seemed:
‘any philosopher's argument which does not therapeutically treat human suffering is worthless. Just as there is no profit in medicine when it doesn't expel the diseases of the body, so there is no profit in philosophy when it doesn't expel the suffering of the mind.' 
Yoga continues to be explored by many people, precisely because of its power to expel the suffering of the mind. I could change the quotation a little and I think it would make perfect sense in a Yoga context:
‘any yoga practice, which does not therapeutically treat human suffering, is worthless. Just as there is no profit in medicine when it doesn't expel the diseases of the body, so there is no profit in yoga when it doesn't expel the suffering of the mind.'
This is how we might measure the benefits of practising yoga. TKV Desikachar expressed it this way: 'The success of Yoga does not lie in the ability to perform postures but in how it positively changes the way we live our life and our relationships.'
 Kaṭha Upaniṣad (VI.11)
 Bhagavad Gītā (II.48; II.50 & VI.23)
 Yoga Yājñavalkya (I.43-45)
 TKV Desikachar, 1995, The Heart of Yoga, Inner Traditions International: Rochester, Vermont, USA (p.149)
 Yoga Sūtras (I.2-4)
 TKV Desikachar, 1995, The Heart of Yoga, Inner Traditions International: Rochester, Vermont, USA (p. 5, 6, 79)
 Alain De Botton, 2000, The Consolations of Philosophy, Hamish Hamilton: London