 Over the centuries philosophers have suggested various instantiations for the contingent being noted in  premise 1.  In his Summa Theologica (I,q.2,a.3), Aquinas argued that we need a causal explanation for things in motion, things that are caused, and contingent  beings.[1]  Others, such as Richard Swinburne (2004), propose that the contingent being referred to in premise 1 is the universe. The connection between the two is supplied by John Duns Scotus, who argued that even if the essentially ordered causes were infinite, “the whole series of effects would be dependent upon some prior cause” (Scotus [c. 1300] 1964: I,D.2,p.1,q.1,§53). Richard Gale (1999) calls this the “Big Conjunctive Contingent Fact”. Whereas the contingency of particular existents is generally undisputed, not the least because of our mortality, the contingency of the universe deserves serious defense (see  section 4.2).  Premise 2 invokes a moderate version of the Principle of Causation, according to which there must be a sufficient cause for any contingent being or event, or of the Principle of Sufficient Reason, according to which there must be a fully adequate explanation for any contingent being or event. Applied here, the Principles of Causation or of Sufficient Reason support the contention that if something is contingent, there must be a sufficient cause of its existence or a fully adequate reason or explanation why it exists rather than not exists. The point of  premise 3  is simply that something cannot cause or explain its own existence, for this would require it to already exist (in a logical if not in a temporal sense).  Premise 4  is true by virtue of the Principle of Excluded Middle: what explains the existence of the contingent being either are solely other contingent beings or include a non-contingent (necessary) being.  Conclusions 6  and  7  follow validly from the respective premises.