I can agree with the statement because If the person knows that the proposition is true then the proposition is certain. For a person to know that a claim is true then the claim must be certain. They can justify their reasoning with the knowledge they already have about the subject. Since the person already has knowledge about the claim and they cannot be wrong about it because of the support they can base their opinion on then the claim must be certain. And since the person knows the claim is true then the claim must be certain, therefore the person will have had "conclusive justification" for the claim. A person will be confident about a subject if they have knowledge about it which would make them confident believing that the proposition is certain whether they agree upon it or not.
This argument is not as complicated as I thought it was from just looking at it. Like the example used on the philosophy press website, someone could truly believe that their horse is going to win without really knowing it will win. But to really know that the horse is going to win, he or she would need more evidence, for example that the race has been fixed. Personally I believe in God and think that He exists but I do not have the physical evidence to prove that he exists except my beliefs. But if I know that God exists then I cannot be wrong that God exists. So doesn’t that mean that it is certain that God exists?
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