The 2,000-Year-Old Advice Psychology Took a Century to Take Seriously

In the 1st century AD, a Roman teacher was already telling other teachers to watch closely for differences in talent among their students. Almost two thousand years later, in 1890, the most important psychology book of its time barely mentioned the word “intelligence.”

Between those two facts lie almost twenty centuries.

And that gap, according to psychologist Arthur Jensen, wasn’t an oversight. It was the symptom of something much deeper about how nearly the entire history of Western civilization understood the human mind.

The advice of a Roman teacher of rhetoric

In The g Factor (1998), Jensen traces the earliest explicit mention he’s been able to find in the philosophical literature of individual differences in mental ability. And it doesn’t come from a scientist, but from an orator.

Marcus Fabius Quintilianus (35-95 AD), a teacher of rhetoric in imperial Rome, wrote advice that, as Jensen points out, wouldn’t feel out of place in a modern textbook on educational psychology:

“It is generally, and not without good reason, regarded as an excellent quality in a teacher to note accurately the differences of ability in those he has undertaken to instruct, and to discover the direction in which the nature of each particularly inclines him. For there is in talent an almost incredible variety, and the forms of the mind are no less…”

Quintilian already took for granted, twenty centuries ago, something academic psychology would take centuries to accept as a serious object of study: that people differ, notably, in their mental capacity, and that a good teacher should know how to recognize it.

An 1890 book that barely uses the word “intelligence”

The contrast Jensen offers next is, if anything, even more striking.

William James, one of the founders of modern psychology, published his work Principles of Psychology in 1890, one of the most influential and comprehensive texts of the discipline in its time.

• The word “intelligence” appears in that book only once, and only as a synonym for “intellect” and “reason,” in the context of defining the general properties of the mind.
• Nowhere in the book is there any mention of individual differences in that capacity.
• The same is true, according to Jensen, of James’s own Talks to Teachers (1899), the first influential textbook of educational psychology in the United States, and of James Mark Baldwin’s Handbook of Psychology (1890), another reference text of the era.

In other words: the most important psychology books of the late 19th century, written with the explicit intent of covering the entire field, barely devote a single line to something that today is one of the discipline’s central subjects.

Why something so obvious took so long to become science

Jensen offers two main explanations for this centuries-long gap, and neither has to do with nobody having noticed it before.

The first is philosophical. Since Plato, the Western tradition (later reinforced by Christian theology) held a sharp separation between body and soul. The soul, in this view, was a universal, perfect, and immaterial quality, the same in essence for all of humanity. That idea was simply incompatible with the notion that some people had more or less mental capacity than others: the soul admitted no degrees.

The second is social. Before industrialization, access to formal education was the privilege of a small elite. Social status at birth determined, almost entirely, each person’s educational and occupational opportunities. In a world like that, individual differences in mental ability stayed hidden behind a far more visible and determining inequality of opportunity.

It wasn’t until the arrival of industrialization, with the spread of specialized occupations and wider access to formal schooling, that these individual differences began to become visible on a large scale, and with them, to become a subject worthy of scientific study.

From Darwin to Spearman: the turning point that changed everything

The turning point, according to Jensen, came with Darwin’s theory of evolution in the mid-19th century. For the first time, the mental capacity of humans and animals was understood as the product of an evolutionary process, subject to individual variation, just like any other physical trait.

From there emerged Herbert Spencer and Francis Galton, who proposed that human intelligence was, to a large extent, a general and heritable trait. Their conclusions, Jensen notes, rested more on conviction than on solid evidence, but they set the questions that would dominate research for the following century.

And it was Charles Spearman, in 1904, who finally found a rigorous way to put that idea to the test: a statistical method, factor analysis, capable of showing that performance on very different mental tasks tended to be correlated, as if a common factor lay behind all of them. He simply called that factor g.

What two millennia of delay teach us about science

What’s interesting about this story isn’t just the anecdote, it’s what it reveals about how scientific knowledge advances, or fails to.

• An observation can be obvious to anyone with common sense, as it was to Quintilian, and still take centuries to become an object of rigorous study.
• The obstacle, often, isn’t a lack of data, it’s a prior framework of ideas (philosophical, religious, or social) that makes that observation literally unthinkable within that framework.
• When that framework shifts (in this case, with the theory of evolution), what had been invisible for centuries suddenly becomes the center of an entire discipline.

What Quintilian observed with plain common sense in the 1st century, science took nearly two thousand years to begin measuring rigorously. And even today, as Jensen himself acknowledges, there’s still no full agreement on what to call the very thing that Roman teacher already knew how to recognize in his students.

That lack of agreement is exactly what What Is Intelligence? Not Even the Experts Agree is about.

Sources

• Jensen, A. R. (1998). The g Factor: The Science of Mental Ability. Praeger.
• James, W. (1890). The Principles of Psychology. Henry Holt and Company.