[…] it is illegitimate to reduce an Equation, by subducting from one Side a Quantity when it is not to be destroyed, or when an equal Quantity is not subducted from the other Side of the Equation:

With roral wash redeem her face, / And prove herself of Titan's race, / And, mounting in loose robes the skies, / Shed light and fragrance as she flies.

For quotations using this term, see Citations:shlick.

a hard problem;  a hard question;  a hard topic