 For Pereira, there are three main types of movement in nature. The simplest is movement of inorganic objects. It is ruled by the principles of perpetual and invariable motion. This movement is merely a local one: for example, the attraction of metal to a magnet. The most complex category is voluntary movement (i.e., the complete freedom of movement of humans). At the medium level of complexity, Pereira located what he called “vital movement”, which describes animal movements. Vital movement includes movement caused by muscular and nervous organs in direct response to interior and exterior stimuli; this movement does not involve will nor any kind of spontaneous decision. Notwithstanding this, Pereira said that vital movement does not consist of invariable movement. Instead, the complexity of movements that can be observed in animals is the result of the complexity of the stimuli or forces that can act on them. To clarify this, Pereira classified four different types of animal movement: i) movements produced as a reaction to objects that are present; ii) movements produced by objects that were present in the past time; iii) movements produced as consequence of some previous instruction; iv) movements resulting from the so-called natural instinct (AM: I.241). The central core of Pereira’s theory on animal behavior is found precisely in the explanation of these four types of movement (see Sánchez Vega 1954: 383–387; Bandrés &amp; Llavona 1992: 161; Rodríguez Pardo 2008: 191).