To Florus 5.24

With these matters resolved, I will briefly show how deeply you lie in ignorance, although until now you were considered most subtle and acute. You say you attribute to nature, not marriage, the crime that passes to offspring, just as, on the other side, you attribute human beings to nature, not to a shameful act. You therefore pretend to praise marriage, to avoid being openly detected as a Manichaean, while criticizing nature, in which you confess evil exists and from which evil is propagated. Could you never hear the rules of argument and the principles of thoroughly sound reason? In every category, genera encompass more than species; subordinate genera also arise in this way. Species encompass more than individuals. Genera contain species, and the lowest species contain individuals. Shaking the lower things, then, does not shake the higher; but the higher things, by their own collapse, carry down everything they encompassed.

For example, ‘animal’ is a genus. In the breadth of its meaning it includes different species, such as human being, horse, ox, and others. If one species perishes, the genus does not suffer destruction. Suppose the nature of oxen disappears from existence: the genus is certainly not destroyed, since the nature of other animals remains. Conversely, if what ‘animal’ means is removed from existence, every species included under that genus undoubtedly perishes. No species of animate creature will remain once animal life is entirely destroyed. The higher things therefore share their fortunes and characteristics with the species they encompass. The reverse does not occur: the higher are not altered by the variations of their species as though by binding relationships.

To apply the example to the case: human nature in its generality is a kind of genus for the arrangements situated within it. It has, as it were, species in bodily position, organs, orders, movements, and other such things. It therefore shares its quality with all its subordinate species, but does not bind itself so as to share the risks of things lower than itself. If nature, then, is condemned and believed guilty and subject to the devil, marriage beneath it, fertility, and the whole substance will also be condemned. Marriage, which follows nature, cannot therefore be praised if nature itself is criticized. The beauty of severed shoots must die together with its root. To put the thing more plainly: the business of marriage cannot be called good if natural union is accused, since what is devalued in the genus to which it inseparably belongs cannot be honored in the species.

But when an evil will uses natural instruments for shameful acts, that power of pleasure and seed—which never varies according to the partners’ will—shares no part in the crime. It supplies material to God as he works, and the shameful act condemns only the adulterer’s deserving conduct, not nature’s. Since we were discussing natural things, your condemnation of nature alongside praise of marriage was exceedingly dull-witted. It is irrefutably established that a genus shares whatever it receives with its species. Therefore either the use the apostle calls natural will be judged good and legitimate, marriages will be honorable, and there will be no natural sin; or, if nature is believed diabolical so that original sin can exist, marital intercourse too will be declared condemnable. This is to embrace the Manichaeans’ doctrine, not soberly, but openly. Since that doctrine is deadly, and they possess nothing of truth, honor, or faith, while the theory of natural sin can belong to no others, it is established that we are catholics and you Manichaeans (compare Romans 1:27).