It must be noted that possible existence is contained in the concept or idea of everything that we clearly and distinctly understand; but in no case is necessary existence so contained, except in the case of the idea of God. Those who carefully attend to this difference between the idea of God and every other idea will undoubtedly perceive that even though our understanding of other things always involves understanding them as if they were existing things, it does not follow that they do exist, but merely that they are capable of existing. For our understanding does not show us that it is necessary for actual existence to be conjoined with their other properties. But, from the fact that we understand that actual existence is necessarily and always conjoined with the other attributes of God, it certainly does follow that God exists.
Whatever may be said against this, it in fact is an adequate response to the objection Byrne raised against it, at least if that objection is developed no more than Byrne develops it; particularly when added to the rest of Descartes's discussion both in the First Replies and in the rest of his Replies to the Objections. Unless we have suddenly decided to overrule arguments by fiat, we have to take into account the actual defenses that have been made against objections before; otherwise we are just repeating things that have been answered before, and moving the discussion nowhere.