Ontological concepts occupy a central position in philosophy. This is primarily because human beings are interested in understanding the nature of existence, the question of why there is "something" rather than "nothing", of enquiring into the meaning of existence, and so forth. Sadr al-Din al-Shirazi (d. 1661), known as Sadra or Mulla Sadra, developed a new philosophical system based totally upon the concept of existence (wujud). The same attempt was made by the German philosopher Martin Heidegger (d. 1976). This book is a comparative case study of both philosophers and demonstrates how, from the Islamic and the Western traditions, they both base their philosophy on the concept of existence.