Probably the most challenging issue in science and advanced technology is the ever increasing complexity. The term complexity refers to the experience that the complex whole is more than the sum of the parts. Emergence of new properties is observed at all levels, from relatively simple physical systems up to high-end evolution in biology or state-of-the-art microprocessors in technology. In this study an effort is made to arrive at an understanding of the underlying ontological basis in terms of the classical (...) philosophy of Aristotle and Aquinas. In addition, the value of philosophy is emphasized as a means to develop the capacity for intuition. Only with this capacity it is possible to acquire an understanding of the great variety of concepts needed in the multidisciplinary approach to complex systems. (shrink)