언제나 그렇듯... 든든합니다, 조지님...
줄리아님도 든든하죠... ㅎㅎㅎ
It's about "VERB + ~~ing" in English....
just like the "~~ing", Korean has the same rule called, "VERB STEM + ~~는 것" or "VERB STEM + ~~는 게" 
Here is an example,
hm... what about... if you want to say, "It'd better start eating 불고기" 
"eating" is not a verb here.  It's "a gerund".  
Its, "eating", translation in Korean is 먹는 것 or 먹는 게.  
How?  why?  
It's simple!  the verb - "to eat" - is 먹다.  먹 is the verb stem, and you add 는 것 or 는 게 => it becomes 먹 + 는 것 or 먹 + 는 게 => 먹는 것 / 먹는 게
What about this then - "seeing is believing"?
What is the verb here?  the main verb is "is"!!!  Both "seeing" & "believing" are gerunds.  
Let's translate the sentence then, 
"seeing" => 보다 "to see".  보 is the verb stem, and you add 는 것 or 는 게 => 보 + 는 것 / 보는 게 => 보는 것 / 보는 게.
"believing" => 믿다 "to believe".  믿 is the verb stem, and you add 는 것 or 는 게 => 믿 + 는 것 / 믿 + 는 게 => 믿는 것 / 믿는 게.  
Now its translation is a piece of cake!!!
"seeing" 보는 것이 or 보는 게
"believing" 믿는 것 or 믿는 게.
Therefore, "seeing is believing" is 보는 것이 믿는 것이다.  (we don't say 믿는 게이다).
I hope my explanation helps,
Thank you all~~~ 
best, 
Tim   
