  As we noted above, Dignāga and, following him, Dharmakīrti  restrict perception to types of cognitive awareness that lack  conceptual determination (kalpanāpoḍha). But even direct perception is not always warranted, given the possibility of  perceptual illusions. For this reason, Dharmakīrti adds a  further constraint, by admitting as reliable only those perceptions  that lack conceptual determination and also are non-erroneous  (abhrānta) (see  Pramāṇaviniścaya I, 252). In fact, by adding the qualifier ‘non-erroneous’ Dharmakīrti is simply  following an established tradition of excluding from perception  instances of perceptual illusions of the sort produced by, say, the  rapid rotation of a firebrand, or cases of sensory impairment as in  cataracts. For perceptions to be reliable their object must be  causally effective. The Buddhist epistemologist solves the problem  of the efficacy of perceptions by adopting a representationalist  stance: perception can only reveal its object by assuming its form.  Thus, an intermediary realm of cognitive aspects  (ākāra) is posited between perception and  conception.