Much ado over must and ought to

I read the letter from Paustinus Siburian (May 23) in which a comparison was made between the use of "must" and "ought to". I agree that the two are different, but I am confused by the analysis put forward.

The verbal auxiliary must has two main uses: as an expression of strong probability or assumption; and as an expression of necessity, compulsion or obligation. In the latter, must corresponds to a command.

On the other hand, ought to, which is used as an expression of duty, rightness, advisability or probability corresponds, at its strongest, to a moral duty. This is weaker than a command. It seems to me, therefore, that the definitions put forward in Siburian's letter have been transposed.

FRANCIS R. DENSON

Jakarta