Hello Hisun, It is an idiom that means people can't believe something if they don't experience it personally, it extends beyond the sense of sight to include experiencing with other senses too. For example : A- Paris is a wonderful city to travel to. B- I have never been there, I can't tell. Seeing is believing. It means unless I experience traveling there I can't tell for sure that you are right or I can't believe you absolutely.

This idiom means that you must believe something that you see (especially something surprising or strange). It's hard to believe something you have not seen. "I'll believe it when I see it with my own eyes."

hello Hisun, The meaning is "you have to see that thing (whatever it is) with your own eyes, before you're agree with everybody... Another expression to say that too is " to be like St Thomas" (être comme St Thomas) in the Bible who was a disciple of Jesus ...

you can't believe (in) a thing unless you see it

hi, it's like saying : "Show me a proof" or "prove it!" (a tangible evidence to establish the truth of something). :)