The movement of animals is explained by Stoics as a composure of impulse (instinct or desire) and impression (imagination and representation).
We may say that such composure follows the Stoic cosmic principles of action and passivity. The impulse functions as an action, while the impression is, at least in its primary stage, a form of passivity.
Though the Stoics assign to man the movement through rational powers, too, it is inviting to think that human life movement is also guided by the same causes as for animals.
We may recognize the desire of living even if its rationale poorly exists. Also, we use impressions and their Stoic cognate, phantasms for always finding reasons of this life and for a supposed after life.
By renouncing to rational pride, it is possible to take the three terms as a whole: movement, desire, and impression. The same implicit understanding should pertain to other animals.
As a consequence, the question about the reasons of life as a primary human movement can be substituted by its components. We may question ourselves about the reasons of our desires and impressions. Both of them exist undoubtable, but less obvious the life as a whole.
Therefore, any answer about impulses and impressions has greater chances to reveal facts about our life than the question about life itself could do.
Even a great question as ‘What is the meaning of our life?’ may receive an answer by means of a description of our impulses and impressions. It would seem unsatisfactory just for the fact that there is necessarily required a long journey in life for depicting them.