What was G.K. Chesterton's proof of original sin, and what is so unique about it? Certain new theologians dispute original sin, which is the only part of Christian theology which can really be proved. - Orthodoxy, by Gilbert K. Chesterton; The Maniac Page 1

What is GKC's proof of original sin, and what is unique about it that similar logic can't be used to prove any other belief?
What was G.K. Chesterton's proof of original sin, and what is so unique about it?
For Chesterton not only is sin a fact, original sin is a fact, an observable fact one can &quot;see in the street.&quot; It is his particular logical outlook on the matter.
Someone once asked Bill Vallicella his opinion on the following passage from G. K. Chesterton:my opinion of the following passage from G. K. Chesterton:


Modern masters of science are much impressed with the need of beginning all inquiry with a fact. The ancient masters of religion were quite equally impressed with that necessity. They began with the fact of sin -- a fact as practical as potatoes. Whether or no man could be washed in miraculous waters, there was no doubt at any rate that he wanted washing. But certain religious leaders in London, not mere materialists, have begun in our day not to deny the highly disputable water, but to deny the indisputable dirt. Certain new theologians dispute original sin, which is the only part of Christian theology which can really be proved. Some followers of the Reverend R. J. Campbell, in their almost too fastidious spirituality, admit divine sinlessness, which they cannot see even in their dreams. But they essentially deny human sin, which they can see in the street. The strongest saints and the strongest sceptics alike took positive evil as the starting-point of their argument.

What Chesterton is saying is that sin is a fact, an indisputable fact, whether or not there is any cure for it. Not only is sin a fact, original sin is a fact, an observable fact one can &quot;see in the street.&quot; Chesterton also appears to be equating sin with positive moral evil.
Is moral evil the same as sin? If yes, then the factuality of moral evil entails the factuality of sin. But it seems to me that moral evil is not the same as sin. It is no doubt true -- analytically true as we say in the trade -- that sins are morally evil; but the converse is by no means self-evident. It is by no means self-evident that every moral evil is a sin.  Let me explain.
Moral evil is evil that comes into the world from a misuse of free will. As such,  it could exist whether or not God exists as long as there are free agents.  All that would be required for the existence of moral evil, in addition to free agents, would be moral values and/or moral laws. Sin, however,  implies God by its very concept. Sin is an offense against God. A sinful act is not just a wrongful act, but an act of disobedience, a contravention of a divine command. From the Catholic Encyclopedia article on sin:

In the Old Testament sin is set forth as an act of disobedience (Genesis 2:16-17; 3:11; Isaiah 1:2-4; Jeremiah 2:32); as an insult to God (Numbers 27:14); as something detested and punished by God (Genesis 3:14-19; Genesis 4:9-16); as injurious to the sinner (Tob., xii, 10); to be expiated by penance (Ps. 1, 19). In the New Testament it is clearly taught in St. Paul that sin is a transgression of the law (Romans 2:23; 5:12-20); a servitude from which we are liberated by grace (Romans 6:16-18); a disobedience (Hebrews 2:2) punished by God (Hebrews 10:26-31). St. John describes sin as an offence to God, a disorder of the will (John 12:43), an iniquity (1 John 3:4-10).

My first conclusion, then, is that moral evil is not the same as sin. The concept of sin includes the concept of moral evil, but not conversely. This is because sin is an offence against God. If so, then it is difficult to see how sin could be a fact, as Chesterton claims. It is more like an interpretation of certain facts. We need an example.
One man brutally assaults another to get his wallet. He beats him to death with a baseball bat while the vitim's little girl looks on in horror.  The act is evil, and let's assume that the act's being evil is a fact not only in the sense that it is the case, but also in the sense that it is evidently the case, observably the case, indisputably the case.  But is the act of assault sinful? Only if God exists. For only if God exists can there be an offence against God, which is what sin is. But that God exists is not a fact in the sense I just defined. For even if it is the case that God exists -- even if the proposition God exists is true -- it is not evidently, observably, indisputably the case that God exists. Chesterton says one can &quot;see sin in the street.&quot;  This is just false. For surely one cannot see God in the street, or in the sky, or in nature as a whole. The theist interprets what he literally sees in terms of, within the horizon of, his belief in God, and so he interprets the evil act as a sinful act. But the sinfulness of the act of assault is not a perceptible quality of it: it cannot be 'read off' the act.
My second conclusion, therefore, is that sin is not a fact in the sense defined. This is because calling an act sinful  involves an interpretation of the act in terms of an entity, God, whose existence is not a fact in the sense defined. It is interesting to note that if sin were an observable fact, then, given that concept of sin include the concept of God,  we would be able to mount a quick argument for God from the existence of sin. That is, we could argue as follows:
There are sinful acts; If there are sinful acts, then God exists; ergo, God exists. This argument is valid in point of logical form, but is not probative because it begs the question in the first premise: anyone who classifies some acts as sinful in so doing presupposes the existence of God.
So, contrary to what Chesterton says above, sin is not a fact one can &quot;see in the street.&quot; It is no more an observable fact than the createdness or divine designedness of the universe  are observable facts. They may be facts, but they are not observable facts. I seem to recall Kierkegaard saying something similar to what Chesterton says above.  Kierkegaard, if memory serves, says in effect that Original Sin is the one dogma that is empirically verifiable. But this is the same mistake. The most one can say is that the fact of moral evil is plausibly explained by the doctrine of Original Sin. If the doctrine is true, then we have a plausible explanation of the ubiquity and horrendous depth of moral evil; but other explanations are possible which operate without theistic assumptions.- Is Sin a Fact? A Passage from Chesterton Examined