Now we come to Asana, posture, the part of yoga most people know. Patanjali's whole description of it is just one short line: posture is that which is firm and pleasant.

That is it. He doesn't list dozens of poses or say you must touch your toes. He says your posture should be steady, so you don't fidget, and comfortable, so you can stay in it.

In Patanjali's time, the main purpose of posture was to sit still for meditation. If your back hurts or your legs fall asleep, your mind will keep thinking about your body instead of becoming calm.

This is good news for everyone. You don't have to be flexible or young to do this kind of yoga. Sitting upright on a chair with your feet flat on the floor is a perfectly good posture.

Modern yoga classes with many poses can be wonderful for health. But Patanjali's test is simple: can you sit steady and at ease?

Try it now: sit up tall, let your shoulders relax, rest your hands on your knees and stay still for one minute.