We have discussed the vicissitudes of certain internal stimuli related to deficits. We have called the deficit itself a need and have called the stimulus arising from the need a craving. These cravings initially are kept from upsetting the organism's psychic integrity through variations in the height of threshold mechanisms. Certain cravings eventually achieve psychic representation and then become impulses which can be dealt with through psychic defenses based on displacement mechanisms. Cravings are different from impulses on three counts: (1) they are protected against by thresholds rather than psychic defenses; (2) they lack quality which impulses possess; (3) because of the all-or-none basis on which threshold mechanisms operate, once insistent cravings gain access to consciousness they are likely to disrupt the immature ego's fragile synthetic efforts.

We thus have postulated that there are at least two levels of drive