  Descartes’ actual reply to this objection, which he took very seriously, is highly complex and couched in terms of a theory of “true and immutable natures.” We can simplify matters by focusing on its key elements. One of his first moves is to introduce a point that we discussed earlier (see passage [5] in section 2), namely that existence is contained in the idea of every thing that we clearly and distinctly perceive: possible (or dependent) existence is contained in our clear and distinct idea of every finite thing and necessary (or independent) existence is uniquely contained in the idea of God (AT 7:117; CSM 2:83). So for Descartes one does not have to build existence into the idea of something if that idea is clear and distinct; existence is already included in every clear and distinct idea. But it does not follow that the thing represented by such an idea actually exists, except in the case of God. We cannot produce ontological arguments for finite things for the simple reason that the clear and distinct ideas of them contain merely dependent existence. Actual existence is demanded only by the idea of God, which uniquely contains independent existence.