If you had been blind all your life and could suddenly see, could you distinguish by sight what you knew already by touch—say, a cube from a sphere? Would flowers look like flowers you'd felt and faces like faces, or would they all be confusing patterns? How would you start to make sense of the many objects in your immediate view? If we are born knowing nothing, how do we come to know anything?

Harvard University psychologist Elizabeth S. Spelke takes these questions to the people who may be best able to answer them: babies. Spelke, whose sprawling laboratory in William James Hall teems with infants and researchers who are interested in them, has addressed some of the most intractable mysteries of human knowledge by interrogating little people who cannot yet talk, walk or even crawl. She has what she calls “an insatiable appetite” for assessing these young beings. Through Web pages, flyers and letters to day care centers and pediatricians' offices, her lab mates ask anyone and everyone for diminutive volunteers. They watch as the little subjects sit on their mothers' laps, tracking the stagecraft that Spelke and her cohorts use to gauge early understanding of numbers, language, objects, space and movement.

Spelke's findings have helped revise sharply our notion of what humans can make sense of in their first days, weeks and months. In doing so, she has offered some of the most substantial evidence to date regarding nature versus nurture. Spelke's discoveries about infant capabilities have become central to ongoing attempts to figure out human cognition.

From her insights she has forged a bold, if still controversial, theory of “core knowledge,” which asserts that all humans are born with basic cognitive skills that let them make sense of the world. This core knowledge, Spelke says, underlies everything we learn throughout our lives and both unifies and distinguishes us as a species. Her theory prompted the American Psychological Association to honor her with its William James Fellow Award in 2000. And her work shows that, despite people's differences, we all have more in common than we recognize.

Clarity, Not Confusion
The heart of Spelke's methodology is her observation of “preferential looking”—the tendency of infants and children to peer longer at something that is new, surprising or different. Show a baby a toy bunny again and again, and the baby will give it a shorter gaze each time. But give the bunny four ears on, say, its tenth appearance, and if the baby looks longer, you know the baby can discern four from two. The approach neatly bypasses infants' deficiencies in speech or directed movement and makes the most of the one thing they control well: how much time they fix their eyes on an object.

Spelke did not invent the scheme of studying preferential looking. That credit falls to Robert L. Fantz, a Case Western Reserve University psychologist who in the 1950s and early 1960s discovered that chimps and infants stare longer at things they perceive as unexpected. A researcher could gauge an infant's discriminatory and perceptual powers by showing the baby different, highly controlled scenarios, usually within a stagelike box, and observing what changes in the scenarios the infant would perceive as novel.

Using this basic technique, Fantz and others soon found that the infant's world was not, as pioneering psychologist William James had opined in 1890, a “blooming, buzzing confusion.” Infants made sense of the world readily. For example, Fantz and others found that newborns could differentiate red from green, two-month-olds could discriminate all primary colors, and three-month-olds preferred yellow and red to blue and green. They found that a newborn could distinguish between her mother's face and a stranger's (unless both adults wore scarves over their hair), a four-month-old could recognize acquaintances, and a six-month-old could interpret facial expressions. By the 1970s psychologists recognized the first year of life as a far more explosive developmental period than they had ever considered it to be.

This work attracted Spelke when she was still an undergraduate at Radcliffe College. From 1967 to 1971, she studied with Harvard child developmental psychologist Jerome Kagan and quickly found herself hooked on the excitement of investigating the essential workings of human cognition by analyzing children. She continued that research while pursuing her Ph.D. in psychology at Cornell University, where famed developmental psychologist Eleanor J. Gibson served as her graduate adviser and mentor. Gibson, one of only a handful of psychologists to win the National Medal of Science, had revealed much about infant cognition with some elegant experiments of her own. Her best known was the “visual cliff,” a piece of heavy glass extending from a tabletop. Would early crawlers avoid the apparent drop-off? Most do, a discovery that revised theories of infants' spatial understanding.

Under such tutelage, Spelke hit on her own landmark experiment. “At dinner one night,” she recalls as we talk in her office at Harvard, “I was musing with a fellow student over whether, when babies look at and listen to something, they perceive [the sight and sound of an event] as two separate things, or do they recognize a link between the two? How would you find that out? Suddenly I had this image of two visual events going on side by side, like movies, and between them a loudspeaker that you could switch from the sound of one event to the sound of the other event. Would a baby turn to look at the event matching the soundtrack the speaker was playing? That experiment became my Ph.D. thesis. It was the first time I was able to start with a general question about how we organize a unitary world from multiple modalities and turn the question into a ridiculously simple preferential-looking experiment—which actually ended up working.”

Sure enough, Spelke found that babies recognized the link between sound and sight, switching their gaze back and forth as the soundtrack changed. Thus began Spelke's career of pondering big questions with straightforward experiments on tiny people. The mixed-modality approach addressed the same “binding problem” faced by blind people who suddenly can see: How does the brain mesh the signals from different senses into a single impression? Spelke did not answer how, but she did show persuasively that this ability seems innate.

Native Knowledge
Over the years Spelke has conjured up many other elegant and productive investigations on object and facial recognition, motion, spatial navigation, and numerosity (grasping of numerical relationships). She is able to envision simple but powerful tests, she says, “because I think like a three-year-old.” By showing babies objects in motion and then interrupting their logical speed or course, she has found that even a four-month-old infers that a moving object is supposed to keep moving. Yet it takes an eight-month-old to grasp the principle of inertia and expect the object's path to be consistent and smooth.

By showing babies different arrays of disks, she has found that six-month-olds can distinguish eight from 16 and 16 from 32—but not eight from 12 or 16 from 24. By having babies watch a person reach for one of two objects on a table, she has found that although 12-month-olds know from an adult's gaze which object the person will grab, eight-month-old babies do not.

As the data from such clever designs mounted, Spelke began to develop her theory of core knowledge, often inspired by or collaborating with colleagues such as noted Massachusetts Institute of Technology linguist Noam Chomsky, French mathematician turned cognitive neuropsychologist Stanislaus Dehaene and Harvard psychologist Susan Carey. Core knowledge systems, Spelke says, are neuronal “modules” that are in place at birth for building mental representations of objects, persons, spatial relationships and numerosity. Somewhat akin to the “deep grammar” that Chomsky believes underlies all human language, these core knowledge modules enable all infants to organize their perceptions.

The sophistication of these systems in infants resembles that of modules in nonhuman primates, suggesting an ancient, evolutionary development; a six-month-old baby understands numbers, space, objects and faces much as a mature rhesus monkey does. As Spelke sees it, these cognitive tools underlie all the more complex skills and knowledge we master as we grow up—spoken languages, number manipulation and other abstract mental operations. Core knowledge forms the basis for the robust cognitive machinery that gets us through life. And we almost completely ignore it.

“Even for adults,” Spelke says, “most of what we know that lets us negotiate the world, guide our choice of paths through the environment, understand whether a car down the street might hit us or whether a falling object will miss us, even what we say as we're conversing—most of that is completely unconscious. How many things do we do that we hardly think about? Most of what we do is like that. We operate on richly structured cognitive systems that aren't usually accessible to introspection. To me, this is one more sign that most of our cognitive workings are much like those of babies and are built on the core knowledge that we had as babies.”

Equality of the Sexes
This view of Spelke's is what philosophers call a “nativist” theory—that certain of our traits are inborn. They are natural rather than nurtured. Spelke knows well that this puts her on a slippery slope. To speak of native abilities is to court speculation about native differences in those abilities. In early 2005 Spelke found herself involved in a hot controversy about such possible differences when she was repeatedly asked for her opinion of Harvard president Lawrence Summers's remarks, made that January, that biological disparities might help explain why women occupy so few places in university math and science departments.

Spelke, of course, was the natural choice to debate this topic, not only because she was a prominent, highly accomplished scientist at Summers's university but because she got there by studying precisely the innate abilities Summers wondered about. Although she hardly seems a scrapper by inclination, Spelke is quick-witted, funny, impressively well informed and eminently agile in conversation. And she rose quite gracefully to the task of popping Summers's thought balloon.

“If you look at things Summers's way,” she says in her office, leaning forward in her chair with a sly grin, “then to study innate cognitive abilities, like I do, is supposedly to study gender differences. In fact, I didn't know we were studying gender differences at all, because we don't find any. But since the subject came up”—she spreads her hands, clasps them, then sits back in her chair, smiling—“I was happy to tell him about our work.”

Summers got an earful, if not directly, as Spelke described in several interviews and in a high-profile public debate with her colleague and friend Harvard psychologist Steven Pinker how voluminous evidence from decades of research shows little if any inherently sex-based differences in infants or toddlers. At those early ages, when culture has the least effect but sex hormone levels are extremely high, no sex-based differences have shown themselves in a huge variety of skills that underlie mathematical thinking. For example: put a four-year-old in a distinctly shaped room, hide a block in a corner, have the four-year-old close his eyes and spin around, then have the child hunt for the block. Some of the children will quickly reorient themselves in the room and find the object, whereas others will not. Yet the percentages of boys and girls who succeed are identical. So although “there is a biological foundation to mathematical and scientific reasoning,” as Spelke put it in her debate with Pinker, “these systems develop equally in males and females.”

Spelke, an unabashed optimist, believes our growing understanding of cognitive abilities will eventually reduce, rather than inspire, divisions about our human qualities. “This idea that we have native abilities,” she tells me, “some find threatening, for it seems to invite the idea that some types of people might be innately better endowed than others. If you're a nativist about basic core cognitive capacities, as I am, does that also lead you to be a nativist about, say, differences among the sexes? These claims of biological bases can proliferate to a point where they end up being invoked to explain everything. But you have to be very careful about what data you use.” The information that seems to indicate sex differences, Spelke says, comes from problematic studies whose results are colored by cultural influences—everything from parents responding differently to girls and boys to university faculties viewing identical job applications more skeptically when the applicant's name is female. Summers must have taken that last point to heart: in May 2005 he announced that Harvard would spend $50 million over 10 years to recruit and support women and minorities on its faculty.

Meanwhile the expanding pile of data on infants, who are not tainted by culture, shows remarkable parity among sexes and races. “We're getting evidence for an intricate and rich system of core knowledge that everyone shares and that gives us common ground,” Spelke declares. “In a world of so much conflict, I think that's something we badly need.”