This chapter presents a reasoned discussion of the notion of life according to an intuitive conception of life, which is moreover in conformity with the way it was usually characterized by pre-modern philosophers and scientists whose imagination was not particularly biased by the mechanical view of the world. It will be argued that this conception is in line with many contemporary scientists’ intuition of the notion of life. This conception of life will form the basis of our study of living systems throughout the rest of the book.