Ought implies can

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Proposition

"...as much as is possible...more no man can be obliged."

And therefore, to promise that which is known to be impossible is no covenant. But if that prove impossible afterwards, which before was thought possible, the covenant is valid and bindeth, though not to the thing itself, yet to the value; or, if that also be impossible, to the unfeigned endeavour of performing as much as is possible, for to more no man can be obliged.

Hobbes, Lev XIV 25