Seeing the world through a philosophical point of view, concepts such as toughness or rigidity are emergent properties of metalic materials that result from the complex dynamic behaviour of its components. Also dynamics of population of dislocations are closely related to the population dynamics of every entity, despite the big difference in nature and the behaviour of the components, a population of interacting entities, will dispay same collective behaviour as long as the interactions are non linear and also as long as the population operates in a non thermodynamic equilibrium.