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Many voice hearers simply think that they are insane. It is in fact quite easy to determine wether a voice is created by an intelligent person or being. If you select words randomly from a dictionary and string them together you will almost certainly not get a sentence. If you do this repeatedly you will likely not get any sentences. If your brain malfunctions and words arise randomly from your memories you would expect the same type of random jibberish. You might get some remembered phrases or sentences arising from your memories, but you would not get any type of intelligent conversation. A defect involving the part of the brain that composes sentences might allow for the random creation of reasonably well formed sentences, but they would tend to be complete nonsense nonetheless. It will be extremely unlikely that you will get any intelligent sentences in an ongoing manner, not even the pointless degrading voices many suffer from.