Abstract: Some concepts are pointed out, including the definition of the soil skeleton, its composition and the space occupied by the soil skeleton. It is indicated that soil mechanics is a subject to study the mechanical properties of the soil skeleton and its influencing factors. A large number of examples with some serious consequences induced by collapse of the soil skeleton are illustrated. Based on the analysis of forces on particles, the mechanism of seepage force in the soil skeleton is explained, and the formula for calculating the seepage force is derived. In particular, that generally the so-called “seepage force is the drag force of the soil skeleton by flow of water” is not comprehensive.