Memory itself is still something of a mystery, but it basically consists of physical changes in the brain that encode a representation of past experiences. Those memory traces—known as engrams—can be accessed to reconstruct the past, albeit imperfectly. Many experts believe that engrams are built by strengthening synapses—the sites where signals are transmitted between nerve cells, or neurons. Recalling a memory reactivates a pattern of nerve-cell signaling that mimics the original experience.

“The prevailing view is that the formation of an engram involves strengthening of synaptic connections between populations of neurons … that are active during an event,” Sheena Josselyn and Paul Frankland write in the current Annual Review of Neuroscience. “This increases the likelihood that the same (or similar) activity pattern within this cell assembly can be recreated at a later time.”

Engrams obviously do not save every detail of every experience. Some records of activity patterns do not persist. And that’s a good thing, says Wimber.

“An overly precise memory is maybe not really what we want in the long term, because it prevents us from using our memories to generalize them to new situations,” Wimber said in San Diego at a recent meeting of the Society for Neuroscience. “If our memories are too precise and overfitted, then we can’t actually use them to … make predictions about future situations.”

If your memory stores every exact detail of getting bitten by a dog in the park, for instance, then you wouldn’t necessarily know to beware of a different dog in a different park. “In fact,” Wimber says, “what we might want is a more flexible and more generalized memory, and that would involve a bit of forgetting of the details and more the development of a gist of a memory.”

Read: Imagining the future is just another form of memory

Such “streamlined” memories are not side effects of flaws or constraints on memory power, Frankland and Blake Richards point out in a 2017 paper in Neuron. Such simplification “is an essential component of adaptive memory,” they write. “Simple memories that store the gist of our experiences and avoid complicated details will be better for generalizing to future events.”

Getting the gist, and just the gist, is therefore valuable as an aid to making smart decisions, say Frankland, of the Hospital for Sick Children in Toronto, and Richards, of the University of Toronto. In fact, they believe it is wrong to think of memory “simply as a means for high-fidelity transmission of information through time.” Rather, they propose that “the goal of memory is to guide intelligent decision making.”

Getting just the gist is especially helpful in changing environments, where loss of some memories improves decision making in several ways. For one thing, forgetting can eliminate outdated information that would hamper sound judgment. And memories that reproduce the past too faithfully can impair the ability to imagine differing futures, making behavior too inflexible to cope with changing conditions. Failure to forget can result in the persistence of unwanted or debilitating memories, as with post-traumatic stress disorder.