Conditions on how to obtain indirect speech acts (e.g. the conditions that one can convey a request by asserting a speaker-based sincerity condition such as the speaker's desire to get something done, or by questioning a hearer-based sincerity condition such as the hearer's ability to do what the speaker wants him to do) are formalized as conversational postulates to be incorporated in a natural logic. Then the need for conversational postulates and transderivational constraints to account for some rules of grammar, is demonstrated.