Many years ago, when I was a Senior Programmer with a consulting company, one of my clients commented that I never said anything was impossible – I never said "No, it cannot be done."
Now of course there are times in my career I have said no. And when I said things could be done, I also noted the costs – which is a way of saying "I can do this, but boy is it going to cost you or disappoint you." However the fact is that in your career, "it cannot be done" is not something you want to say to anyone – including yourself.