Fasting can be done for a limited time to solve the problem of indigestion, which is a medical remedy. Hurting yourself in Nivrutti is also foolish like fasting for the sake of God. However, hurting yourself required in the divine service is appreciable, which is a sacrifice. Hanuman wanted to commit suicide, when Sita was not found. The logic given by Him was that on hearing the negative report on Sita, Rama will not be alive. Therefore, hurting the self here has logical requirement. If somebody tries to commit suicide without any such requirement for the sake of God, it is a foolish sin. Fasting can be a natural consequence of absorption in God. It is appreciable because the hunger will not be experienced at all by you. Jesus and Sai Baba fasted for sometime and such fasting was not forcible since they were absorbed in God.
Forcible fasting can be detected by the feeling of hunger.
Shankara told a simile supporting His reply. A knife can cut the vegetables but not a stone. The stone is not at all cut and on the other hand the edge of the knife is spoiled (shilaaprayukta kshuraadivat…). The knife is the ignorant rules of the ignorant priests. The vegetables are the general ignorant public. The stone is the scholar knowing the whole truth. If you think that burning a dead body in the backyard of the house is inauspicious, burning the dried grass present in the husk in the backyard of the house is equally inauspicious. The green grass has life and hence, the dried grass is a dead body.
The chemical combination of a human body reveals that it contains some inert elements and inert compounds, which are generally found in the materials of the world. Science is the authority as far as Pravrutti is concerned. Shankara is a true scientist in Pravrutti and hence, burnt the dead body like dried grass in the backyard of the home. Science is not related to God and hence, God is irrelevant to Pravrutti as long as none hurts none. The difference between the scientist and Shankara is that the scientist keeps silent about God, whereas the Shankara stressed on the existence of God. Therefore, Nivrutti should not be dragged into Pravrutti as long as the basic principle is not violated.