Final answer:
The fading of memory over time as described in the question is best categorized as the sin of memory called 'transience', which represents the gradual loss of memory detail.
Step-by-step explanation:
The fading of memory with the passage of time marks decay and is best exemplified by the sin of memory known as transience. Transience refers to the tendency of memories to degrade over time, which is why we may struggle to remember details of past events such as the books we've read. For instance, if a student named Nathan's mother cannot remember the plot of 'To Kill a Mockingbird' except for a few character names, despite having read it in high school, this reflects memory transience. This is different from, for example, absentmindedness, which is a lapse in memory due to a break in attention or being focused elsewhere, or blocking, which is an inability to retrieve information that is on the tip of your tongue.