A complex system cannot be properly described at a single level, like for example in the case of systems where microscopic interactions give rise to macroscopic patterns. The number of levels is not necessarily limited to two: For example, in the description of Brownian motion along the lines of classical physics we identify three levels: a microscopic one (molecular level), where the motion is deterministic (in a classical mechanics account) and time-reversible; a mesoscopic level, i.e. that of the Brownian particle, where the motion is stochastic and irreversibility has appeared, and a macroscopic one, exemplified e.g. by a billiard ball moving in water, whose motion is deterministic and irreversible. This example also shows that levels have their own autonomy, although higher levels are rooted in the lower ones, and that each level needs to be described with the methods appropriate to that level.