A narrator suffering from a condition such as Alzheimer’s Disease might provide a valid, though unreliable narrative. I would consider using this if I wanted my narrator to fail to recall a crime s/he committed or perhaps an unpleasant act that resulted in damaging, long-term consequences. Then again, the narrator might not yet have been diagnosed and false ideas could be expounded by a narrator trying not to appear ‘mad’, or in a detective story fail to report something vital to the police. Because this state of mind is often not consistent, the author might choose to create a narrator who deliberately shows forgetfulness over, say, past actions they wish to keep secret, whilst also actually forgetting some truths shown to the reader by another narrator or in a prologue, for example. The excuse of loss of memory could result in the narrator lying to the reader. This is an example of the device that I would like to experiment with.