All things exist in the middle between two extremes.
Aristotle invented a concept called the Golden Mean, a middle ground between two extremes. Aristotle considered that the best path was always the middle way between two extremes.
The human body is a complexity of many parts, which is its form, but with an overall design to “become” through the process of living and growth. There is a delicate balance between form and design in the human body, for without a heart (form) the body will die, and if the heart fails to beat (design) the body will die. The balance between the form of heart and the design of heart is the middle, and if either move beyond that middle ground their relationship separates, and death is the result.
Always the body exists in the middle between two extremes, a range of measures from temperature to acidity, to which in extreme will separate the form from the design. A heart may beat too fast or too slow, and by doing so either way cause a collapse in the entire body, thus the middle way is the aim in the body at all times through the mediation of Logos.
A river has design in it flows, and form that it is liquid. The river is subject to the middle course, for if through heat all the water evaporates it no longer has form, thus it ceases to exist, just as cold freezes its motion into ice, and thus impacts design. In both extremes of temperature the form and design in a river will separate, thus the middle is the preferred course to exist as a river.