Primary mental abilities were first discussed by L.L. Thurstone in 1935 and could be described as latent core constructs that can explain nearly all cognitive differences.

Thurstone found that when measuring intelligence, there are only a handful of aspects that really matter. He found the following aspects:
1. Verbal comprehension - knowledge of vocabulary and in reading
2. Number facility - solving simple arithmetical computation and reasoning problems
3. Word fluency - writing and producing words
4. spatial visualization (mentally visualizing and manipulating objects),
5. inductive reasoning (completing a number series or predicting the future based on past experience),
6. associative memory (remembering people’s names or faces), and
7. perceptual speed (rapidly proofreading to discover typographical errors in a typed text)

The primary mental abilities are organised into clusters known as Secondary Mental abilities – two major components, proposed by Cattell, include fluid and crystallised intelligence.

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Sources:
Encyclopedia of applied psychology
http://education-portal.com/academy/lesson/primary-secondary-mental-abilities-definition-examples.html