He has to show not the law, but that the law applies.

Can he convince me?
then he gains the cause.

And thus you find Christ giving various counsels to varying people, and often jealously careful to avoid definite precept.

Is he asked, for example, to divide a heritage?
He refuses: and the best advice that he will offer is but a paraphrase of that tenth commandment which figures so strangely among the rest.

_Take heed, and beware of covetousness_.