Why did Gardner propose a different definition?

Gardner worked with brain-damaged individuals and noticed some bizarre effects.

He observed people with normal IQ's but who had no memory of the past, or couldn't add, or were unable to use common nouns in speech, or were unable to recognize faces.

He decided that the traditional notion of intelligence was too narrow. Instead of a single intelligence represented by a single number, he proposed that there are 8 different intelligences.