Saucha means purity; physical and environmental cleanliness and mental purity.
(2.40) Purity contributes to good health and avoidance of disease.
(2.41) The clarity of sattwic influences [in the mind and awareness] contributes to serenity, cheerfulness, purposefulness, mastery of the senses, improved powers of concentration, and receptivity to Self-realization.   
Patanjali´s YogaSutra (2.40/2.41) Quelle: Roy Eugene Davis “The Science of Self-Realization”