Because of the growth of entropy, we have a very different epistemic access to the past than to the future. In retrodicting the past, we have recourse to "memories" and "records," which we can take as mostly-reliable indicators of events that actually happened. But when it comes to the future, the best we can do is extrapolate, without nearly the reliability that we have in reconstructing the past. However -- the human brain, as most readers of this blog probably know, was not intelligently designed. It's doesn't have the high-level structure of a computer program, where all the processes are carefully planned to achieve some goal. (The lower-level structures share the mechanical features of any other physical system, but that's of little help here.) Evolution nudges the genome in useful directions, but it can only work with the raw materials it's given; it doesn't have the luxury of starting from scratch. So over and over in biological organisms, we find features that were originally developed for one purpose being re-engineered for something else. As it turns out, the way that the human brain goes about the task of "remembering the past" is actually very similar to how it goes about "imagining the future." Deep down, these are activities with very different functions and outcomes -- predicting the future is a lot less reliable, for one thing. But in both cases, the brain goes through more or less the same routine.

That's what Daniel Schacter at Harvard and his friends have discovered, by doing functional MRI studies of brains subjected to different kinds of cues. (Science Newsreport, Naturereview article, Charlie Rose interview.) Subjects are inserted gently into the giant magnetic field, then asked to either conjure up a memory or imagine a future scenario about some particular cue-word. What you see is that the same sites in the brain light up in both cases. The brain on the left in this image is remembering the past -- on the right, it's concocting an imaginary scenario about the future.

Further confirmation comes from studies of amnesiacs, who famously can't remember the past. But if you ask the right questions, you find that they also have significant problems imagining their own future. We tend to assume that the brain must be like a computer -- when we want to access a memory, we simply pull up a "file" stored somewhere on the brain's hard drive, and take a look at its contents. But that's not it at all. Schacter believes that pieces of data relevant to any particular memory -- times, images, sounds -- are stored piecemeal in different parts of the brain. When we want to "remember" something, another part of the brain assembles these pieces into a (hopefully) coherent picture. It's like running a new simulation every time you need a memory, and it's the same thing we do when we try to imagine some event in the future. Everyone has heard that memories can be unreliable, but many of us don't appreciate the extent to which that is true. It's not the case that "real" memories are stored once and for all deep in the darkest recesses of the brain, and it's just a matter of digging them up. False memories -- conjured from any number of sources, from gradual embellishment to direct suggestion by others -- seem precisely as vivid and real to us as accurate memories do. For a good reason: the brain uses the same tools to construct the memory from the available raw materials. A novel and a history book look the same on the printed page.