Editing Alvin Plantinga (section)

=== Modal ontological argument ===

Plantinga has expressed a [[Ontological argument#Alvin Plantinga|modal logic version of the ontological argument]] in which he uses [[modal logic]] to develop, in a more rigorous and formal way, [[Norman Malcolm]]'s and [[Charles Hartshorne]]'s modal [[Anselm's argument|ontological arguments]].

Plantinga criticized Malcolm's and Hartshorne's arguments, and offered an alternative.<ref>{{cite news |url=http://chronicle.com/article/Evolution-Shibboleths-and/64990/ |title=Evolution, Shibboleths, and Philosophers |newspaper=[[The Chronicle of Higher Education]] |date=April 11, 2010 |access-date=2010-04-28 |quote=Like any Christian (and indeed any theist), I believe that the world has been created by God, and hence "intelligently designed"}}</ref> He argued that, if Malcolm does prove the necessary existence of the greatest possible being, it follows that there is a being which exists in all worlds whose greatness in ''some'' worlds is not surpassed. It does not, he argued, demonstrate that such a being has unsurpassed greatness in this world.<ref name="Plantinga, Sennett 1998">{{Cite book |title=The Analytic Theist: An Alvin Plantinga Reader |last=Plantinga |first=Alvin |publisher=Wm. B. Eerdmans Publishing |year=1998 |isbn=978-0-8028-4229-9 |editor-last=Sennett |editor-first=James F. |pages=65–71 |author-link=Alvin Plantinga}}</ref>

In an attempt to resolve this problem, Plantinga differentiated between "greatness" and "excellence". A being's excellence in a particular world depends only on its properties in that world; a being's greatness depends on its properties in all worlds. Therefore, the greatest possible being must have maximal excellence in every possible world. Plantinga then restated Malcolm's argument, using the concept of "maximal greatness". He argued that it is possible for a being with maximal greatness to exist, so a being with maximal greatness exists in a possible world. If this is the case, then a being with maximal greatness exists in every world, and therefore in this world.<ref name="Plantinga, Sennett 1998" />

The conclusion relies on a form of [[axiom S5|modal axiom S5]], which states that if something is possibly true, then its possibility is necessary (it is possibly true in all worlds). Plantinga's version of S5 suggests that "To say that p is possibly necessarily true is to say that, with regard to one world, it is true at all worlds; but in that case it is true at all worlds, and so it is simply necessary."<ref>{{Cite book |url=https://books.google.com/books?id=veGCAgAAQBAJ&q=%22possibly+necessarily+true%22+then+it+is+necessarily+true+plantinga&pg=PA128 |title=Medieval Philosophy: An Historical and Philosophical Introduction |last=Marenbon |first=John |date=October 2, 2006 |publisher=Routledge |via=Google Books |isbn=978-1-134-46183-7}}</ref> A version of his argument is as follows:<ref name="oppy">{{Cite encyclopedia |url=http://plato.stanford.edu/entries/ontological-arguments |title=Ontological Arguments |last=Oppy, Graham |date=8 February 1996 |encyclopedia=substantive revision 15 July 2011 |publisher=Stanford Encyclopedia of Philosophy}}</ref>

# A being has ''maximal excellence'' in a given possible world ''W'' if and only if it is omnipotent, omniscient and wholly good in ''W''; and
# A being has ''maximal greatness'' if it has maximal excellence in every possible world.
# It is possible that there is a being that has maximal greatness. (Premise)
# Therefore, possibly, it is necessarily true that an omniscient, omnipotent, and perfectly good being exists.
# Therefore, (by axiom S5) it is necessarily true that an omniscient, omnipotent and perfectly good being exists.
# Therefore, an omniscient, omnipotent and perfectly good being exists.

Plantinga argued that, although the first premise is not rationally established, it is not contrary to reason. [[Michael Martin (philosopher)|Michael Martin]] argued that, if certain components of perfection are contradictory, such as omnipotence and omniscience, then the first premise is contrary to reason. Martin also proposed parodies of the argument, suggesting that the existence of anything can be demonstrated with Plantinga's argument, provided it is defined as perfect or special in every possible world.<ref>{{Cite book |title=Philosophy of religion: an anthology |last=Martin, Michael |publisher=Wiley-Blackwell |year=2003 |isbn=978-0-631-21471-7 |pages=282–293}}</ref>

Another Christian philosopher, [[William Lane Craig]], characterizes Plantinga's argument in a slightly different way:
# It is possible that a maximally great being exists.
# If it is possible that a maximally great being exists, then a maximally great being exists in some possible world.
# If a maximally great being exists in some possible world, then it exists in every possible world.
# If a maximally great being exists in every possible world, then it exists in the actual world.
# If a maximally great being exists in the actual world, then a maximally great being exists.
# Therefore, a maximally great being exists.
According to Craig, premises (2)–(5) are relatively uncontroversial among philosophers, but "the epistemic entertainability of premise (1) (or its denial) does not guarantee its metaphysical possibility."<ref>{{Cite book |title=Reasonable faith |last=Craig, William Lane |publisher=Crossway |year=2008 |isbn=978-1-4335-0115-9 |page=185 |quote=Premises (2)–(5) of this argument are relatively uncontroversial. Most philosophers would agree that if God's existence is even possible, then he must exist. ... the epistemic entertainability of premise (1) (or its denial) does not guarantee its metaphysical possibility.}}</ref> Furthermore, [[Richard M. Gale]] argued that premise three, the "possibility premise", [[begs the question]]. He stated that one only has the epistemic right to accept the premise if one understands the nested [[modal operator]]s, and that if one understands them within the system S5—without which the argument fails—then one understands that "possibly necessarily" is in essence the same as "necessarily".<ref>{{Cite book |title=On the Nature and Existence of God |last=Gale |first=Richard |publisher=Cambridge University Press |year=1993 |isbn=0-521-45723-8 |page=227}}</ref> Thus the premise begs the question because the conclusion is embedded within it.
On S5 systems in general, James Garson writes that "the words 'necessarily' and 'possibly', have many different uses. So the acceptability of axioms for modal logic depends on which of these uses we have in mind."<ref>{{Cite book |last=Garson |first=James |date=February 29, 2000 |title=Modal Logic |publisher=Metaphysics Research Lab, Stanford University |url=https://plato.stanford.edu/archives/sum2014/entries/logic-modal/ |via=plato.stanford.edu}}</ref>