Review early psychological theories regarding intelligence. Charles Spearman considered intelligence to be a general factor. He called it the "G factor" or simply "G" when developing formulas which he thought could improve our understanding of intelligence. He applied the variable "S" to specific intelligence, which people develop with personal experience. Spearman held personal experience to be an incredibly important factor when defining intelligence.
For example, if two students (A and B) are learning to drive a car and student A is offered 10 hours of practice time, while student B is offered 2 hours of practice time, it is probable that student A will perform better when driving skills are tested. Student A will have developed more "S", the specific skill for driving. To Spearman, this doesn't mean student A is is smarter than B. Subject A just has more experience.
Review L.L. Thurstone's definition of human intelligence. Thurstone defined intelligence as seven separate mental abilities: spatial, perceptual speed, numerical talent, verbal meaning, memory, reasoning and word fluency.
Consider more modern definitions of intelligence, such as Steinberg's Triarchic Theory. You can think of Steinberg's Triarchic Theory like a three-pronged approach to the definition of intelligence. His theory divides intelligence into three main types: analytical, creative and practical. Steinberg defined analytical intelligence as the ability to gain knowledge and solve problems. He theorized that standard intelligence tests only prove this single type of intelligence. He defined creative intelligence as the ability to adapt, or to change a situation to your liking. On his definition of practical intelligence, Steinberg stated that individuals with practical intelligence are good at finding solutions to personal problems. They seek out situations that match their skills, and are able to shape those situations to match their abilities. They also know when and how to change situations to better fit them.
Understand how modern intelligence tests define human intelligence. The Stanford-Binet Intelligence Scale, for example, is used to estimate and define overall intelligence. The test is made of 15 subtests. These are designed to measure four types of mental abilities considered components of human intelligence: verbal reasoning, abstract/visual reasoning, quantitative reasoning and short-term memory.