The ontological goodness of things is their rational desirability. Therefore, an evil is “a missing good’, a privation ( I, 48,1), the absence of good that ‘ought’, either naturally or morally, to be there,. It is privation, like blindness in a person, not mere absence like blindness in a stone. Natural evils, like animal blindness, suffering and death, lethal earthquakes, tornadoes, and the like, are only incidentally evil, that is, locally and relatively undesirable by affected creatures, (if they are not caused, like some plane crashes, say, by immoral acts).
Nothing, in so far as it has being, is or can be evil. It is not possible for God to make something that is less than it ‘ought’ to be. For in so far as it is made, the thingis rationally desirable. “Nothing can be essentially bad”(I, 49, 3). Further, there is nothing that absolutely ought not to be, not even the worst evil actions of free rational creatures. Still (I, 49,2), God can be the cause of what ought not to be, by causing a penalty fitting to justice (I, 49,1), for creaturely wrongs, but never can, as angels can (I, 63,1), and humans can, cause evil by fault (I, 48,5).“Every evil in voluntary things is to be looked on as pain or fault”(I, 48,5).
He reasons that it is within the perfection of a divine agent to make a created order in which the natural and imperfect causes produce effects that people deem evils, like plagues, pestilences, bugs and beasts that harm us; for, of course, finite causes will be imperfect agents. Therefore, the “order of nature requires that some things can, and sometimes do, fail” (I, 49,2). At I, 22, 2, ad 2, he says, “hence, corruption and defects in natural things are said to be contrary to some particular nature; yet they are in keeping with the plan of universal nature”; and he reasons that “a lion would cease to live if there were no slaying of animals”, thus acknowledging that biotic life requires death, that it is not a defect for God to create carnivorous and vicious animals, and poisonous snakes. Human suffering and death is another matter because it is the result of Adam’s sin which incurred the loss of an extra-natural divine protection in which human persons were first created (I, 97,1).