"Success comes in cans, failure in can'ts".

- Author unknown

I remember as a kid my grandpa reading me bedtime stories when I stayed at their house.  One I remember as a small kid was about the train engine trying to make it up the hill; you remember, where the train engine kept saying:

"I think I can, I think I can, I think I can"

and then as he topped the hill, he started saying:

"I knew I could, I knew I could, I knew I could"

That is the essence of this quote, think you can and you will!  Think you can't, and you wont!