Understanding is a wellspring of life unto him that hath it: but the instruction of fools is folly.
Understanding is a well-spring of life unto him that hath it - "a well-spring," whence flow counsels, informations, consolations, tending to spiritual life.
But the instruction of fools is folly - "instruction" (Hebrew, musar); rather, discipline or castigation. As "understanding" or intelligence is its own reward to its possessors, so "folly" is its own punishment to fools. Or, taking the English version, All "the instruction (i:e., the wisdom) of fools is (nothing but) folly;" it is "a well-spring" of folly, and therefore of death (the opposite to "of life"). Water cannot rise above its level. Not only in serious concerns does the wise man's "understanding" well forth, but even in his moments of relaxation. The fool's wisdom, not only in ordinary concerns, but even in serious ones; as, for instance, his efforts by way of "instruction" betray his "folly."