They were of the belief that his blindness was caused by sin. Jesus had explained that this blindness was not because of sin but for the glory of God. They had no rational rebuttal against the man's argument, so they resorted to their fall back position - if all else falls blame it on someone else's sin. This locks them into an indefensible position that only brute force can resolve. We should be cautious of this line of argument as well.
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