<P> In the world, we can see that things are caused . But it is not possible for something to be the cause of itself because this would entail that it exists prior to itself, which is a contradiction . If that by which it is caused is itself caused, then it too must have a cause . But this cannot be an infinitely long chain, so, therefore, there must be a cause which is not itself caused by anything further . This everyone understands to be God . </P> <P> As in the First Way, the causes Aquinas has in mind are not sequential events, but rather simultaneously existing dependency relationships: Aristotle's efficient cause . For example, plant growth depends on sunlight, which depends on gravity, which depends on mass . Aquinas is not arguing for a cause that is first in a sequence, but rather first in a hierarchy: a principal cause, rather than a derivative cause . </P> <P> In the world we see things that are possible to be and possible not to be . In other words, perishable things . But if everything were contingent and thus capable of going out of existence, then, given infinite time, this possibility would be realized and nothing would exist now . But things clearly do exist now . Therefore, there must be something that is imperishable: a necessary being . This everyone understands to be God . </P> <P> The argument begins with the observation that things around us come into and go out of existence: animals die, buildings are destroyed, etc . But if everything were like this, then, at some time nothing would exist . Some interpreters read Aquinas to mean that assuming an infinite past, all possibilities would be realized and everything would go out of existence . Since this is clearly not the case, then there must be at least one thing that does not have the possibility of going out of existence . However, this explanation seems to involve the fallacy of composition (quantifier shift). Moreover, it does not seem to be in keeping with Aquinas' principle that, among natural things, the destruction of one thing is always the generation of another . Alternatively, one could read Aquinas to be arguing as follows: if there is eternal change, so that things are eternally being generated and corrupted, and since an eternal effect requires an eternal cause (just as a necessary conclusion requires necessary premises), then there must exist an eternal agent which can account for the eternity of generation and corruption . To hold the alternative, namely that an infinite series of contingent causes would be able to explain eternal generation and corruption would posit a circular argument: Why is there eternal generation and corruption? Because there is an eternal series of causes which are being generated and corrupted . And why is there an infinite series of causes which are being generated and corrupted? Because there is eternal generation and corruption . Since such an explanation is not acceptable, there must be (at least one) eternal and necessary being . </P>

St thomas aquinas 5 causes of the universe