This is the chief virtue of doers like Elon Mush; they understand the distinction. The average person—someone like me—notes what Musk is trying to do and shakes their head with a mixture of exasperation and envy. “Colonise Mars? Harvest the Sun’s energy and use it to eradicate the incumbent fossil fuel industries? Develop brain-computer-interfaces and make human enhancement viable? Impossible.” Not really.
Something that is “impossible” cannot be achieved, no matter the time, energy, attention, or resources directed at the task, either now or at any point in the future. Something that is “too hard” cannot be achieved with the current allocation of time, energy, attention and resources.
Some things that are impossible include the ability for anything to travel faster than the speed of light (there is speculation and conjecture, but nothing has yet been proved substantive) and the violation of certain universal laws (unless we discover these laws are wrong). Some things that are too-hard-but-not-impossible include time travel (really) and the ability for humans to grow wings (gene modification, extrapolated).
Of course, the things Musk is trying to do and ideas like violation of universal laws, time travel and human flight are particularly lofty examples of the distinction between “too hard” and “impossible.” Instead, consider how this distinction applies to the more mundane activities that make up the majority of our lives. If I’m struggling to understand a concept, it’s not that my brain cannot process the idea ever. I’m just not in a position to do it right now. Think you’ll never be happy, or get a good job, or defeat the black dog of depression? Forget it. To be happy, get a good job and not be depressed is eminently doable, no matter how tall the stack of obstacles in our path seems, but only if we remember that “too hard” is not the same as “impossible.”