Disagree. If I wanted to breathe every 5 seconds and swim 4ish kms under water whilst breathing that water, I could not.
in the context of ironman. yes…to an extent.
i still stand by my statement that ‘anyone can finish ironman…it is all mental.’
as was pointed out by another poster…there is an extent to the human body…but…as a wise man once said…difficult shouldn’t be too hard…impossible might take a little bit longer 
“give me a chance to shine and i’ll blind the whole world.”
I think that the intent of the statement is true, although it is not absolute. (How little sense does that make? )
The point is that if you decide that you want to do something, and really work for it, there is a good chance you’ll succeed. Such is the nature of motivational statements. To apply ridiculous cases to such statements(“Well, if I set my mind to it, can I flap my arms and fly to the moon?”) is silly. It’s all about being invested in your efforts.
Most people can’t do things because they have decided they can’t, or because they aren’t willing to endure what is necessary to achieve their goals.
So yes, I’d say it’s true.
I agree.
Why?
Well, obviously there are limitations to every human being’s mind. If I put my mind to being 7 feet tall, I probably won’t be able to do it, right? Well, maybe I can. Maybe I can figure out how to start my body’s natural growth process even though I’m 30. 
In the context of IM, I think that if anyone is willing to be a pro: eat, drink, train and sleep like a pro; they can accomplish their dream goals. As for simply completing an ironman, I’m quite certain that anyone can do that with enough training. ANYTHING IS POSSIBLE!
Remember, the laws of physics and commen sense were meant to be broken (or at least bent a bit).
-J
I think it is more truthful to say “If you think you cannot do it, you will be correct.”
My mother used to tell me I could be anything I wanted to be, within reason. When I asked her what “within reason” meant, she said, “You ask a lot of questions…for a ditch-digger.”
In my childrens Martial Arts classes we teach the meaning of “Indomitable Spirit” to be “the courage to try something new, as long as it’s safe”
to rewrite your quote in the same spirit: “If you just put your mind to it, you can accomplish anything with in reason.” Now I agree that it’s true, for IM or any thing else.
When I first started teaching I thought anyone can accomplish anything they put their mind to. After a few years, it is very clear there are a number of people who are just not as smart as others and will never have the ability to achieve things others can; no matter how hard they work. That may sound cynical for teacher, but it’s a reality.
Not to turn this into a political discussion but that’s why “no child left behind” is a joke. There are people out there who will be left behind regardless of how hard they work.
I think most people could complete an IM under seventeen hrs if the really wanted to and worked towards it. Whether or not most could do it under nine hours is an entirely different scenario.
OT: How about something more concise - “mind over matter!”
I dont mind, so it does not matter! (grin)
Michael
Of course it’s not true.
Just because we can wrap words around a concept and make it sound poetic doesn’t make it true.
Surely, we want to believe it’s true. And, perhaps it is motivating at times.
But, it’s not true.
right - as henry ford i think said: If you think you can or think you can’t, you are probably right. Key word is modifier “probably”, which serves the same function as your mother’s circumscription. As already noted, mind cannot conquer all. However, the thrust of this quote and quote that started thread is that mental game is an important/essential part of physical performance or any challenge in life. That is the zen of tri.