The concept of balance lies at the centre of visual composition. Balance is the establishment of equilibrium through the resolution of visual tension in order to give images harmony.
Balance is achieved through symmetry, where elements of equal weight are placed equidistant from a central point or axis - everything 'falls away' from this centre evenly giving a measured, uniform feeling. The possibilities for symmetrical composition occur more frequently in man-made environments.
Balance is achieved through asymmetry, where elements are placed at distances inversely proportional to their weight from the central point or axis - flow of movement from the centre occurs at different rates, accelerating and decelerating, giving rise to a sense of dynamism.
In practice, the environment rarely provides us with perfect conditions, so the resolution of visual tension from multiple sources cannot be achieved in a formulaic manner, requiring us instead to resort to the use of developed intuition.
Harmony may not be necessary or even desirable in the visual expression of an idea. The unease from a sense of unchecked falling, of suspension, or of a lack of resolution, can increase the intensity with which emotionally-charged content is experienced.
So perhaps it would be better to say that the concept of balance, its presence or absence, lies at the centre of visual composition.