Hatha yoga is a part of raja yoga. Now, let me explain the term hatha yoga. Hatha means ‘vital’, referring both to prana and mind, manas. Yoga means union and harmony. Hatha yoga means mind and vital energy. This body is composed of physical energy or prana and mental energy or mind. There must be a complete balance and harmony between these twin forces within the body. In hatha yoga balance is maintained between the vital and mental forces. If the vital energy is predominant, one becomes very aggressive and violent, and if the mental forces supersede the vital forces, then there is a lot of impractical thinking, a lot of day dreaming, and people go crazy. They become ‘loony’. The balance between the two is very important. Therefore, I personally do not consider hatha yoga to be merely physical.
Our awareness lives in the physical body and we are aware of the physical body first. Therefore, we have to start with physical awareness. But if you are not particularly aware of the physical body, if you have evolved to the mental body, you must start from there.
Hatha yoga concerns itself with the perfection or purification of the different portions of the physical body, while raja yoga pertains to the mind. Remember that if you just sit for meditation with an impure body, a vacillating mind and an imbalanced nervous system, you will not really progress spiritually. So hatha yoga is a part of raja yoga.