Memory is not perfect. Storing a memory and retrieving it later involves both biological and psychological processes, and the relationship between the two is not fully understood. Memories are affected by how a person internalizes events through perceptions, interpretations, and emotions. This can cause a divergence between what is internalized as a memory and what actually happened in reality; it can also cause events to encode incorrectly, or not at all.

Transience

It is easier to remember recent events than those further in the past, and the more we repeat or use information, the more likely it is to enter into long-term memory. However, without use, or with the addition of new memories, old memories can decay. "Transience" refers to the general deterioration of a specific memory over time. Transience is caused by proactive and retroactive interference. Proactive interference is when old information inhibits the ability to remember new information, such as when outdated scientific facts interfere with the ability to remember updated facts. Retroactive interference is when new information inhibits the ability to remember old information, such as when hearing recent news figures, then trying to remember earlier facts and figures.

The Thinker by Auguste Rodin

Our memories are not infallible: over time, without use, memories decay and we lose the ability to retrieve them.

Encoding Failure

Encoding is the process of converting sensory input into a form able to be processed and stored in the memory. However, this process can be impacted by a number of factors, and how well information is encoded affects how well it is able to be recalled later. Memory is associative by nature; commonalities between points of information not only reinforce old memories, but serve to ease the establishment of new ones. The way memories are encoded is personal; it depends on what information an individual considers to be relevant and useful, and how it relates to the individual's vision of reality. All of these factors impact how memories are prioritized and how accessible they will be when they are stored in long-term memory. Information that is considered less relevant or less useful will be harder to recall than memories that are deemed valuable and important. Memories that are encoded poorly or shallowly may not be recoverable at all.