The Third Way: Argument from Contingency

Thomas Aquinas

The argument is valid.

Every inference was checked by machine. Objection O1 stands against P1. So C1 is contested.

Open in the playground

flowchart TD
    classDef axiom fill:#f6efd9,stroke:#a8842a,color:#23212b;
    classDef premise fill:#e7edf7,stroke:#1f4e9c,color:#23212b;
    classDef deduction fill:#f3f4f1,stroke:#5b6b85,color:#23212b;
    classDef conclusion fill:#d6e1f3,stroke:#1f4e9c,stroke-width:3px,color:#23212b;
    classDef objection fill:#f7e3e1,stroke:#b3261e,color:#23212b;
    classDef reply fill:#f3f4f1,stroke:#7a5c2e,color:#23212b;
    classDef imported stroke-dasharray:4 3;
    classDef defeated fill:#f3f4f1,stroke:#b3261e,stroke-dasharray:6 3,color:#8a8796;
    classDef undecided stroke-dasharray:2 2;
    classDef overruled fill:#f3f4f1,stroke:#c9ccc6,color:#8a8796;
    ST(["ST: Thomas Aquinas, Summa Theologiae, I,<br/>q.2, a.3"]):::axiom
    P1["<b>P1</b>: If everything could fail to exist, then<br/>at some time nothing existed.<br/><i>(∀x. Contingent(x)) → NothingOnceExisted</i>"]:::premise
    P2["<b>P2</b>: If at some time nothing existed, nothing<br/>would exist now, since nothing comes<br/>from nothing.<br/><i>NothingOnceExisted → ¬∃x. ExistsNow(x)</i>"]:::premise
    P3["<b>P3</b>: Something exists now.<br/><i>∃x. ExistsNow(x)</i>"]:::premise
    D1["<b>D1</b>: Therefore not everything is contingent:<br/>something exists necessarily.<br/><i>∃x. ¬Contingent(x)</i>"]:::deduction
    P4["<b>P4</b>: Since necessary things cannot borrow<br/>their necessity from one another without<br/>end, some necessary being is necessary<br/>of itself.<br/><i>(∃x. ¬Contingent(x)) → (∃x.<br/>NecessaryOfItself(x))</i>"]:::premise
    P5["<b>P5</b>: A being necessary of itself is what all<br/>call God.<br/><i>∀x. NecessaryOfItself(x) → God(x)</i>"]:::premise
    C1["<b>C1</b>: Therefore God exists.<br/><i>∃x. God(x)</i>"]:::conclusion
    O1{{"<b>O1</b>: Quantifier shift: that each thing fails<br/>to exist at some time does not imply<br/>that there is a time at which everything<br/>fails to exist."}}:::objection
    R1{{"<b>R1</b>: Over an infinite past every possibility<br/>of non-existence would be realized, and<br/>realized together."}}:::reply
    R2{{"<b>R2</b>: Realized possibilities of non-existence<br/>need not coincide; an infinite past only<br/>guarantees each occurs at some time<br/>(Anthony Kenny, The Five Ways, ch. 4)."}}:::reply
    class C1 defeated
    class D1 defeated
    class P1 defeated
    class R1 overruled
    ST -.-> P1
    ST -.-> P2
    ST -.-> P3
    P1 --> D1
    P2 --> D1
    P3 --> D1
    ST -.-> P4
    ST -.-> P5
    D1 --> C1
    P4 --> C1
    P5 --> C1
    O1 -. objects .-> P1
    R1 -. replies .-> O1
    R2 -. replies .-> R1

Sources

ST

Thomas Aquinas, Summa Theologiae, I, q.2, a.3

The argument

Objections and replies