We use the word “clever” every day: about a toddler who quickly works out how to get a ball back from under the sofa, about a teenager who defends an opinion with good arguments, about a grandmother who can calm any quarrel. Yet if you ask scientists what intelligence really is, you will hear several different answers. This is not a weakness of science. It is because intelligence is not something you can touch, weigh or photograph.
Why intelligence is so hard to define
Intelligence is a quality of behaviour, not a thing. We never see it directly; we see only what a person does — how they solve a problem, explain an idea or adapt to a new situation. Even a test does not measure intelligence itself: it records answers to tasks and draws conclusions from them, so it measures intelligence indirectly. That is why, as the psychologist Sulaiman al-Khodari al-Sheikh of Ain Shams University in Cairo shows in his book on individual differences, definitions of intelligence fall into several families: philosophical, biological, psychological and operational. Today broad modern theories have joined them. Each family looks at the same quality from its own side.
From Plato to Piaget
The oldest answers come from philosophy. Plato believed the human soul has three parts — reason, courage and desire — and compared it to a chariot in which reason, the driver, guides two horses. Aristotle drew a distinction that still matters: between what lies in a person as a possibility and what has already shown itself in action. This was one of the earliest forms of the idea of “ability”. Many centuries later, Ibn Sina (980–1037) described the “inner senses” in his Book of Healing — common sense, imagination, memory and others. This was one of the early models of the mind's faculties.
Biologists saw intelligence as adaptation to the environment. In the 19th century Herbert Spencer regarded it as a more flexible form of adaptation that gradually takes over from instinct. The Swiss scientist Jean Piaget developed this idea furthest. In his view, a child constantly does two things: fits new experience into existing schemes (assimilation) and changes those schemes when the experience does not fit (accommodation). When a toddler calls a cow a “big dog”, that is assimilation; when they realise a cow is a different animal and adjust the scheme, that is accommodation. For Piaget, intelligence is the most flexible and at the same time the most stable balance between these two processes.
From the first tests to broader views
Psychologists of the early 20th century looked for working definitions. Some called intelligence the ability to learn, others the ability to adapt to new situations; Lewis Terman emphasised abstract thinking. Alfred Binet, the author of the first intelligence test, gave no formal definition, but spoke of the capacity to judge soundly and understand well. Defining intelligence as “the ability to learn” had a weak point: being linked to success at school does not make it the same thing.
In 1923 the American psychologist Edwin Boring proposed an operational definition that is still quoted: “intelligence is what the tests test”. It sounds witty, but it goes round in a circle: the test measures intelligence, and intelligence is whatever the test measures. It says nothing about whether the test itself is any good. Still, it proved useful: it turned researchers towards correlation and factor analysis — studying how results on different tests relate to each other.
Modern theories widen the view. Howard Gardner (1983) defines intelligence as the ability to solve problems or create products that are valued in a culture. Robert Sternberg (1985) sees it as the ability to adapt to an environment, to shape it and, when necessary, to choose a different one. A pupil who gets used to a strict teacher adapts; one who sets up a study group with classmates shapes the environment; one who moves from football to a chess club and blossoms there chooses a new one.
Intelligence A, B and C: what parents can see
The psychologist Donald Hebb proposed a useful distinction. Intelligence A is inborn potential, which cannot be observed directly. Intelligence B is what we actually see in behaviour: the result of that potential meeting the environment — family, school, language, experience. Philip Vernon added Intelligence C: a test score, which is only a small sample of B, taken on a particular day under particular conditions.
So parents never see Intelligence A. They see B every day — at home, at play, with friends — and C only occasionally, in a test report. A test result is information, not a verdict.
What this means in practice
- Watch your child in many situations: in lessons, at play, doing household tasks, with other people. One situation shows only one side of them.
- Notice what absorbs your child: what they do for a long time without being pushed, what they often ask about, what they return to again and again.
- Do not judge a child by one number or one subject. A low mark in maths says nothing about drawing, building things or understanding people.
- Remember that intelligence develops: new experience, good questions and patient support change what a child can do.
- Compare your observations with the teacher's: together you will see more.