You should not decide upon something based just on outward appearances.
- outward appearance cannot be an indicator of someone or something’s value or worth
- you cannot know what someone or something is like just by looking the person or thing’s appearance
- the quality or character of someone or something cannot be judged just by looking at them
- an opinion of someone or something cannot be formed solely by seeing what’s on the surface
- The candidate did not look very intelligent, but you can’t judge a book by its cover.
- The hotel looked attractive from outside, but the rooms were damp and not well maintained. You can’t judge a book by its cover!
- Do not form opinions of a persons character by looking at his appearance. You can’t judge a book by its cover.
- At first we did not want to go into the restaurant as it looked small and cramped; but the food was delicious – we realized that you can’t judge a book by its cover.
- The car was small and looked unsuited for a long trip, but packed a lot of power and was very solid. You can’t judge a book by its cover.
- That man may look very small and insignificant, but don’t judge a book by its cover – he’s a very powerful man in his circle.
This phrase has been in use since the mid 1800s.
*Original from http://www.theidioms.com/