Iorek Byrnison: Can is not the same as must.
Lyra Silvertongue: But if you must and you can, then there's no excuse.

"If you can't you must, and if you must
you can"..

If you can’t, you must. If you must, you can.

But neither can you condemn nor justify and yet be extraordinarily alive as you walk on.
You can never invite the wind but you must leave the window open.

There is no excuse for not trying.

What one can be, one must be!

The fact that you can only do a little is no excuse for doing nothing.

Quand on veut on peut, quand on peut on doit.

If you think you can you can, if you think you can't you're right!

"When people say, 'You really, really must" do something, it means you don't really have to. [...] When it's true, it doesn't need to be said.

You can't. You just have to.

No time is no excuse

Must is the word... You can not fail if you resolutely determine that you will not... Always bear in mind that your resolution to succeed is more important that any other thing.

In our world, you sink or swim, and there are no excuses.