We are part of, and thus cannot be objective about, our own existence. Existence transcends objective enquiry, and is thus a mystery. Scientific questions can be presented to us as problems for which there can be solutions. But philosophic questions are centered on the study of mystery. Philosophy cannot find solutions to questions which cannot be formulated as problems.
Philosophy is thus a mode of enquiry, rather than a problem-solving activity.
Marcel distinguishes between problems and mysteries. A problem is something which can be viewed objecively, and for which we can find a possible solution. A mystery is something which we cannot view objectively, because we cannot separate ourselves from it. Science is concerned with problems about which we can be objective. On the other hand, philosophy is concerned with mysteries, which we cannot stand apart from, or look at objectively.