Why do we forget things? To survive.

Salon on Sunday published an excerpt of David DiSalvo's new book, "What Makes Your Brain Happy and Why You Should Do the Opposite," and it's a fascinating dive into the science of forgetfulness.

First: we know that the brain is a rather flexible organ, the very opposite of a static hard drive in that an array of synapses continually reshape the circuitry of our brains. And from that, we know that neuron activity corresponds to memory -- the more firing between a given set of neurons, the stronger the connection. It's why some memories are stronger than others. The activity between those neurons is more robust.

But most memories aren't as sticky as, say, the moments surrounding the terrorist attacks on Sept. 11, 2001. Over time, they fade. Why?

DiSalvo explains:

The bad news is that our memories are anything but concrete and can be altered with relative ease. The good news is that imperfect memory is an evolutionary adaptation that serves our species well much of the time. Loss of memory, and creation of new memory, is central to a relatively efficient system of information processing that never sleeps. The selective movement of information into long-term memory is an adaptive marvel that allows our brains to store crucial pieces of information that we will rely on in the future, and shed information not worth holding onto.

Which is why memory can be so spotty: biologically, we're continually in the process of spring cleaning, discarding useless information and preserving the most valuable. What our phone number, social security number or spouse's birthday is in modern times was a source of water, food or shelter in our hunter-gathering days. Only in this case, forgetting isn't an angry spouse; it's death.

As we all know, the rise of information technology has tasked our brain with a relentless assault of things to remember. ("Pick a new password. You cannot use any of your previous five.") We're trying to adapt, but we're only human; our brains, therefore, are discarding much of this information, arguably faster than it ever has in history.

The side effects of this are fascinating. For one, we often believe we actually did things we merely observed; second, we fail to scrutinize sources of information once we deem them credible and trustworthy.

The lesson: our memories can be influenced more than we think, and we as humans easily develop false beliefs -- DiSalvo offers demonic possession as an example -- underscored by very real emotions, based solely off of false memories.

Most animals are reactionary, but humans are unique in that we engage in what's called "episodic future thinking," or future simulation through the use of past elements, DiSalvo writes. Because of this dynamic -- the ability to construct possible scenarios of how an event might play out -- imagination occasionally trumps instinct. And thus we're very susceptible to outside influence.

I'd add more, but I seem to have forgotten what else I intended to write. Pity.

This post was originally published on Smartplanet.com