It's pretty easy for me to reflect on the past or dream about the future, but what is the present? How does the human brain perceive the length of the present? Do we live only in the present, or is part of us always in the past and part in the future? —Craig Schneider, Jacksonville, Fla.
Experience tells me, Craig, that questions like these tend to arise while under a certain kind of influence. In case your attention span is currently as short as I suspect, let's just say you're onto something: What we think of as the present doesn't really exist, so it's impossible for us to live in it. Grab some munchies and stay focused, though, and we'll discuss further.
Let's start with physiology. Studies suggest that for us to simply notice something in our field of vision and shift our eyes toward it takes at minimum a tenth of a second. If lightning strikes 100 feet away—a near-definitional example of something that seems to happen "right now"—the bolt will already have changed shape or disappeared by the time you register it and interpret what you've seen. The sensory input that forms our consciousness is itself shaped by the limits of our neural hardware, meaning that what we experience as the present is actually the very recent past.
Your next question is probably: who cares? Our consciousness can arbitrarily define the present as being a very short time in the past and leave it at that. And essentially this is what we do semantically, too—"the present" is a meaningful term to us, even though the thing it refers to isn't something we can actually perceive. Both Aristotle and Saint Augustine saw the present as no more than a single mathematical point, of zero size and duration, separating the past from the future. Philosophy students will be relieved to learn that I concur.
The more interesting part of your question is how and why we can even contemplate the past and the future. This capacity for so-called mental time-travel is considered to be one of the hallmarks of human intelligence.
Animals generally react via instinct. After some experience they can develop behaviors—recognizing a person, playing fetch—that seem to indicate they remember prior experiences. But that's a long way from recalling specifics of the past. While it's obviously very difficult to tell what goes on in, e.g., a cat's brain (it appears to usually be some variant of "fuck you"), humans, as far as we know, are the only animals able to retain literally useless information—knowing the state capitals or the lyrics to "Shake It Off" can't confer much survival advantage. More crucially, it may well be that only humans have episodic memory—i.e., reconstructed knowledge of past events based on one's own perceptions.
The same holds true for the future: Natural selection can result in animal behaviors that appear predictive, but really represent the high survival rate of animals that made similar decisions in generations past. OK, there's weak evidence showing scrub jays, monkeys and rats have some ability to assess the future, but, 1. There's weak evidence showing a lot of things, and, 2. Several studies have also reported that apes do unexpectedly poorly in tasks requiring foresight. From what we can tell, the ability to perform "future simulations"—predictive judgments about future outcomes based on hypothetical situations created in our own brains—is a talent exclusive to humanity. I may like both ketchup and ice cream, but I can guess that a ketchup-flavored ice cream startup won't get much funding. Let's see a scrub jay do that.
Various blobby pictures of brains have indicated that the region responsible for prediction is called the prefrontal cortex. Injury victims who sustain damage to this region may suffer the Oliver Sacksian fate of being "locked in the present." If asked what they're doing tomorrow, these patients draw a complete blank—the concept of "tomorrow" is no longer within their comprehension.
Our vision of the future is also heavily influenced by our recall of the past; research has found links between episodic memory and foresight. The hippocampus has been shown to help us create and store mental maps of our environment, and these maps of the past are later reconstructed to make predictions for the future. Amnesiac patients therefore not only have trouble remembering the past, but also struggle to predict simple future outcomes as well.
The concept of the future is advanced enough that even humans with healthy brains don't acquire it until age three or four, and some studies suggest it doesn't fully develop until age 25, which may explain so many young adults' willingness to take on debt to get a journalism degree. Even in maturity we have confounding tendencies—for one thing, humans tend to be overly optimistic. People suffering from depression, numerous researchers have reported, aren't actually pessimistic in their predictions, just accurate.
J-school students, stoners—none of us are particularly good at fortune-telling. But our ability to imagine the future, even incorrectly, is what makes us human. The present is just the pause while we decide what to do next.
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