The word yoga means joining. When we study what yoga teaches us, we find that it is not a process of joining something with something else, but rather, a process of disjoining or disconnecting two entities that are as though united. We take the two entities to be one and are not able to distinguish between them. Yoga teaches that this perceived oneness of the two entities is the cause of all the pain and sorrow that we have in our lives.
Let us understand this with the help of a traditional example. When a ball of iron is heated in a furnace, it becomes red-hot and appears as a ball of fire. But this ball of fire, which is hot and round, is in fact a union or yoga of two separate entities, iron and fire. Iron is one thing and fire, something else. Each has its own properties. The ball of iron is round in shape, black in color and cold to the touch. Fire is red in color, hot to the touch and has no form or shape.
When iron and fire unite in a fireball, the qualities of each are passed onto or imposed upon the other. The ball of iron, which is black in nature and cold, becomes hot and turns red. Fire, which does not have a form of its own, seems to acquire a round shape. There appears to be a transformation in the iron taking on the properties of fire and fire in turn, taking on the shape of the ball of iron. Both entities unite to become one entity called a fireball. A person who does not know what iron is and what fire is, may take the fireball to be one entity and think that there is indeed a red-hot ball of fire. But there cannot be a ball of fire because fire does not have a form.
A misunderstanding can thus be created if we do not know that what appears to be one, is in fact a union or yoga of two. When we look at that fireball with the knowledge that it is in fact made up of two entities, a process of separation takes place in our mind. We then recognize that the round form belongs to the iron and not to the fire, and that the red color and heat belong to the fire and not the iron ball. So, while the fire appears to be round it remains formless and even while the iron appears to be red and hot, it continues to be black and cold. When this fireball cools down, the iron again becomes cold and black as before and the fire merges back into its formless nature. This separation should take place in our minds in order for us to understand what iron is and what fire is. That is what yoga explains to us.