Whether you think you can, or think you can't... you're right.
Please, stop and think about that saying for a second. What does it mean to you? For the context of this blog, I will address it in the context of weight training. However, I really think it applies to many aspects of life. (Weird that weight training has so many carryovers to regular life.)
Let's say your 1 rep max deadlift is 315 pounds. Thats a serious weight for you. Today is the day that you are going to try and stick a new 1RM. Your training partners load up 340 for you...a 25 pound PR. No joke. The music is loud. The atmosphere is good. Your partners are cheering you on. What's going through your head? If you're not thinking "I'm going to fucking dominate this weight, and it's going to go up like a feather" then you already missed the lift.
If you think you can do it, you're gonna succeed. Let self-doubt creep in your mind for a second, and you're toast. That bar isn't going to make it past your knees.
It's a really good way to learn how important being self-confident is. Is it really possible that the power of thought is that strong that it can be the deciding factor in whether or not you make that lift? You bet your sweet ass it is.
Being strong isn't just a physical thing. You need to be mentally strong in the weight room. Everything sucks while you're doing it, but the benefits will pay off.
Now, PLEASE, go lift something heavy!