Actio immanens - as many other terms, coined by the Aristotelian-Thomist philosophical tradition - is a biological concept par excellence. It was formed as a mental result of biological observation, on the strength of studies on living beings and so, refers to them first and foremost. During the last century, the term actio immanens gradually disappeared from philosophical encyclopedias and has totally vanished from the biological and philosophical language used to describe the dynamism of life. Moreover, if this term does (...) appear at all, its meaning is rather vague. However, actio immanens belongs to the group of key concepts, without which it would seem not possible to properly describe, nor to properly understand biological phenomena. (shrink)