What are mental categories representing activities?

Concepts

Explanation:

People used concepts in order to separate our knowledge into several  categories so it become easier for us to understand and memorize.

We form this concept by finding out similar characteristics that we can observe from different topics, objects or situations around us.

For example , when someone say the word 'car' you would automatically imagine an object with wheels, doors, and box like shape. You do this because you have develop a concept of what the cars look like so the imagery automatically occurs simply by hearing the word.

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