Complex sentence: His point of departure is not a theory, or an idea, but an experience, the experience of despair, which Shestov describes as the loss of certainties, the loss of freedom, the loss of the meaning of life.
Split into simpler sentences: His point of departure is not a theory, or an idea, but an experience. It is the experience of despair, which Shestov describes as the loss of certainties, the loss of freedom, the loss of the meaning of life.