  The third fundamental principle is Soul.  Soul is not the principle of life, for the activity of Intellect is the highest activity of life.  Plotinus associates life with desire.  But in the highest life, the life of Intellect, where we find the highest form of desire, that desire is eternally satisfied by contemplation of the One through the entire array of Forms that are internal to it. Soul is the principle of desire for objects that are external to the agent of desire.  Everything with a soul, from human beings to the most insignificant plant, acts to satisfy desire.  This desire requires it to seek things that are external to it, such as food. Even a desire for sleep, for example, is a desire for a state other than the state which the living thing currently is in.  Cognitive desires, for example, the desire to know, are desires for that which is currently not present to the agent.  A desire to procreate is, as Plato pointed out, a desire for immortality.  Soul explains, as unchangeable Intellect could not, the deficiency that is implicit in the fact of desiring.