Q.Ryan and Shireen are siblings brought up in the same environment. Ryan knows all the formulae in mathematics and can recall them as they are but cannot solve problems based on those formulae, whereas Shireen can use formulae to solve any kind of problem that are based on them. Explain the intelligence they both exhibit. Which approach is this theory based on?

Intellectual Variation — First Encounter

You walk into a room full of people. Some are solving a puzzle in seconds, others take minutes. Some remember a phone number after hearing it once, others need to write it down. Some can hold three ideas in their head at once, others struggle with two.

That difference — in how people think, reason, remember, and solve problems — is what we call intellectual variation.

It is not about "smart" versus "not smart." It is about the fact that human minds differ in kind as well as in degree. Two people can both be brilliant, but one might be lightning-fast with arithmetic and slow with abstract patterns, while the other is the reverse. Variation is the rule, not the exception.

The Intuition

Think of physical traits. Height varies. Eye colour varies. So does the speed at which you can run a 100-metre dash. Intellectual traits are no different. Working memory capacity, processing speed, verbal fluency, spatial reasoning — all of these are distributed across a population. Most people cluster around an average, and fewer people are at the extremes.

But here is the key: these traits are not perfectly correlated. A person with a large working memory may have slow processing speed. A person with fast processing may have poor verbal fluency. The variation is multidimensional.

The Precise Statement

Intellectual variation refers to the natural, measurable differences among individuals in cognitive abilities — including but not limited to:

• Working memory capacity — how much information you can hold and manipulate at once
• Processing speed — how quickly you can take in, interpret, and respond to information
• Verbal ability — vocabulary, comprehension, and verbal reasoning
• Spatial ability — visualising and manipulating objects in space
• Logical reasoning — drawing valid inferences from given premises
• Executive function — planning, inhibition, and cognitive flexibility

These differences are:

1. Quantitative — they exist on a continuum (not "has it" vs "doesn't") …