Imagine that each time you wanted to remember a past experience you had to stop and make a clear decision and a commitment to remember. You would then move on and make a rough description, sketching what you wanted to remember. And once that was done, you would start looking for a memory fitting the description. Clearly, conscious recollections are sometimes the result of such explicit decisions and plans. We sometimes look for particular memories, and we sometimes succeed in finding what we are looking for. But imagine that this were the only possible way in which you could recollect your personal past. What a laborious and inflexible system that would be.

Fortunately, our memory is not just driven by conscious goals and commitments to remember. Often memories of past events come to mind in a manner that is completely unexpected and involuntary. They come with no preceding decision to remember, with no plans and no commitment. They may suddenly pop up in response to stimuli in our environment or aspects of our current thought.