Among demonstrations of proving existence of God is the way of heart or fitra (nature), which is an intellectual attempt on the basis of Quranic essentials in order to make God's existence be proved easier. Islamic thinkers, however, have presented different analyses of fitra and the knowledge of God being natural. Presenting various definitions and analyses, the present essay attempts to investigate their strength and weakness points. As a result of the research, this demonstration, as an independent one, is treated as undefended in its all facets; rather, first it is interpreted as the Demonstration of the Most-truthful formulated by Mulla Sadra on the basis of the existence and then Quranic verses are explained on that ground.