From Sharath Jois Astanga Yoga Anusthana:   Sauca means maintaining both internal and external purity ( antara sauca and bahir sauca , respectively) Thinking good positive thoughts will keep our mind pure. Likewise keeping our environment and person, clothes, body, belongings and surroundings clean and clear will also keep our vision clean and clear. With purity of speech and thought, the mind will also be purified. Once the mind is purified then the atma  (the soul) will be purified. This applies especially to the yoga practitioner, who should keep himself clean in respect to and in consideration for others, and also as an example to others. saucatsvangajugupsa parairasmsargah YS 2.40 From cleanliness, an aversion to one's own body and contact with the bodies of others develops.