Once the source has been discredited, consideration of their evidence and arguments are unnecesary.
When the qualifications and credentials of the source are given as the assurance of proper method, and found to be lacking, then the method is indeed questioned as well as the results.
In addition, we have also considered the evidence and arguments. But you just ignore all that because it's so much easier to blame your car wreck of an argument on the hardness of the wall rather than upon your inability to drive.
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