The origin of the present reflection is to be found in the doctrine I call universal naturalism: everything that exists is natural whether or not it is described and explained satisfactorily by the natural sciences. Now it is well known that despite the progress of the positive and empirical sciences such as physics, chemistry and biology, we do not have a satisfactory explanation of what makes a living being precisely that, a living entity. It is therefore essential to re-examine the thesis of the vital principle, and I believe that contemporary science would honour itself by taking it up again in order to better understand the constitution and behaviour of living beings.