Sattva guna defines the purity of thoughts. On physical level its expression is in the form of calm and composed behaviour and a tranquil mind set. Due to its purity sattva is always referred to as guna. It is never spoiled and hence never called as a dosha.
On the other hand, rajas and tamas are called as ‘maanas’ doshas. They are considered to be the triggering factors that cause mental illness, hence have been termed as doshas.
The diseased body affects the mind which is a cause of feeling irritated and fatigued when one suffers from an ailment. Vice versa occurs when a person is not mentally fit, he tends to feel fatigued and may complain of other symptoms which are related to thought process only.
This is the main tool in ayurveda while making a diagnosis. Exploring the cause is done on the basis of doshas but elaboration of the extent of the disease is made possible by defining gunas only. Rajas guna dominant individual shows aggression in behaviour and is leads an active life. On the other hand a tamas guna individual is generally depressed and has a negative feeling about everything around.
During treatment both of these aspects are considered and applied. Physical well being is affected by mind-set and vice versa. Holistic health which is a combination of healthy mind and body can only be attained when doshas and gunas are in sync with each other and in harmony with the external environment.