A view of death in which the deceased are no longer capable of thinking, acting, or experiencing joys of any kind (9:6, 10). Moreover, they are quickly forgotten by the living (2:16). According to this view, the state of death barely qualifies as existence at all. Qoheleth is either adopting the common view of his times or engaging in his own speculation about the state of death. Either way, his inspired words reveal the limits of reason rather than the actual circumstances attending the souls of the departed. It is only with the deposit of Christian revelation that we learn the full truth about what happens to man after death (particular and universal judgments, resurrection of the body, heaven, hell, etc.)
Also: it is to be understood as referring to the natural knowledge of separated souls, which knowledge is devoid of obscurity in holy men. But he is not speaking of their knowledge in the Word, for it is clear that when Salomon said this, the dead had no such knowledge, since no one had come to the vision of God before Christ's passion.