The 2,000-Year-Old Advice Psychology Took Centuries 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 for psychologist Arthur Jensen that gap wasn’t an oversight, but a symptom of how much of the Western tradition understood the human mind. This article follows his account and adds two pieces he doesn’t mention: a 16th-century Spanish physician and a date that needs putting in order.

The advice of a Roman teacher of rhetoric

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

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

“It is generally, and not without good reason, regarded as an excellent quality in a master to observe accurately differences of ability in those whom he has undertaken to train, and to ascertain in what direction the nature of each particularly inclines him. For there is in talent an incredible variety, and the forms of mind are no less varied than the forms of the body.”

Quintilian, Institutio oratoria, II, 8, 1

Twenty centuries ago, Quintilian already took for granted 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.

A physician in Baeza, fifteen centuries later

Jensen writes from within the English-speaking tradition and jumps from Rome to the 19th century. In between lies a book that is studied in Spain as a landmark of differential psychology: the Examen de ingenios para las ciencias (The Examination of Men’s Wits), by the Navarrese physician Juan Huarte de San Juan, printed in Baeza in 1575.

Huarte took up Quintilian’s intuition and carried it further. He argued that each person has a different “wit,” that these differences have a natural basis (which he explained with the humoral medicine of his time, now long superseded), and that society would gain if each person devoted themselves to the science or trade for which their wit was best suited. That’s why he is cited as a forerunner of educational and vocational guidance.

The book was a huge success, translated into several languages, and it also ran into trouble: the Inquisition censored it, and the revised version was published in 1594, after its author’s death. It’s an interesting nuance for Jensen’s thesis: the idea of differences in ability wasn’t unthinkable before the 19th century, but it was uncomfortable for the dominant religious framework, and there was still no method for putting it to the test.

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 Principles of Psychology in 1890, one of the most influential and comprehensive texts in the discipline at the 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.

The most important psychology books of the late 19th century, written with the explicit aim of covering the entire field, barely devote a 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 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, essentially the same 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 decisive inequality of opportunity.

It wasn’t until 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 Spencer to Spearman: the turning point that changed everything

The turning point, according to Jensen, came with evolutionary thinking in the mid-19th century. The mental capacity of humans and animals began to be understood as the product of an evolutionary process, subject to individual variation, just like any other physical trait.

The dates need putting in order. Herbert Spencer published his Principles of Psychology in 1855, four years before Darwin’s On the Origin of Species (1859): his evolutionism didn’t derive from Darwin, but ran in parallel. Francis Galton, Darwin’s cousin, did build on his work to write Hereditary Genius (1869). Both proposed that human intelligence was, to a large extent, a general and heritable attribute. Their conclusions, Jensen notes, rested more on conviction than on solid evidence, but they set the questions that would dominate research in 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. A year later, in 1905, Alfred Binet and Théodore Simon published the first practical intelligence scale in France, designed for precisely what Quintilian had asked for: identifying which students needed a different kind of teaching.

What two millennia of delay teach us about science

What’s interesting about this story isn’t just the anecdote, but 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, but a prior framework of ideas (philosophical, religious or social) that makes that observation literally unthinkable within it.
• 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 common sense in the 1st century, and Huarte tried to explain in the 16th, 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.
• Huarte de San Juan, J. (1575). Examen de ingenios para las ciencias. Juan Bautista de Montoya.
• James, W. (1890). The Principles of Psychology. Henry Holt and Company.
• Quintilian, M. F. (c. 95). Institutio oratoria, II, 8, 1. English translation by H. E. Butler (1920). Loeb Classical Library, Harvard University Press.
• Spearman, C. (1904). “General intelligence”, objectively determined and measured. American Journal of Psychology, 15(2), 201-292. doi:10.2307/1412107