Kashyut (Kabbalah)

Hardness, or kashyut, is the capacity of the partition to rebuff the supernal light and return it to its source. This is similar to what occurs in the material world: A soft surface absorbs the kinetic energy of objects that are dragged over it, whereas a hard surface will rebuff them. The measure of a partition’s hardness is commensurate with its measure of opacity (oviyut), or the will it contains to receive the supernal light. The greater the measure of the will to receive the supernal light, the greater measure supernal light will be drawn to the created being, and greater the force of the rejection of the light.