tree, I might as well have perceived a bird without a tree or a tree without a bird. This is so because their essences do not depend upon another. David Lewis follows this line of thought in formulating his principle of recombination: "anything can coexist with anything else, at least provided they occupy distinct spatiotemporal positions. Likewise, anything can fail to coexist with anything else". Hume's dictum has been employed in various arguments in contemporary metaphysics. It can be used, for example, as an argument against nomological necessitarianism, the view that the laws of nature are necessary, i.e. are the same in all possible worlds. To see how this might work, consider the case of salt being thrown into a cup of water and subsequently dissolving. This can be described as a series of two events, a throwing-event and a dissolving-event. Necessitarians hold that all possible worlds with the throwing-event also contain a subsequent dissolving-event. But the two events are distinct entities, so according to Hume's dictum, it is possible to have one event without the other. An even wider application is to use Hume's dictum as an axiom of modality to determine which propositions or worlds are possible based on the notion of recombination. A posteriori and necessary truths In Naming and Necessity, Saul Kripke argued that there were a posteriori truths, such as "Hesperus is Phosphoros", or "Water is H2O", that were nonetheless metaphysically necessary. Necessity in theology While many theologians (e.g. Anselm of Canterbury, René Descartes, and Gottfried Leibniz) considered God to be a logically or metaphysically necessary being, Richard Swinburne argued for factual necessity, and Alvin Plantinga argues that God is a causally necessary being. Because a factually or causally necessary being does not exist by logical necessity, it does not exist in all logically possible worlds. Therefore, Swinburne used the term "ultimate brute fact" for the existence of God. See also Ananke Modal logic Platonism A priori and a posteriori References External links http://www.philosophyonline.co.uk/oldsite/tok/rationalism5.htm Modal Metaphysics article in the Internet Encyclopedia of Philosophy Modal Illusions article in the Internet Encyclopedia of Philosophy Necessity Reality