  Both Dignāga and Dharmakīrti take objects of perception to  have causal efficacy (see e.g., PV III, 3). What is generally termed  perception is said to consist in a series of distinct cognitive  events that are causally related. On this model of perception,  contact between sense and object gives rise to a perceptual image  (ākāra) which represents the specific  characteristics of that object. The relationship between the  perceptual image and the empirical object is one of similarity  (sādṛśya). The perceptual image is causally  determined by the object, but the manner of its appearance is  determined by factors that are intrinsic to cognition itself (see,  e.g., PV III, 333–338). Although both Dignāga and  Dharmakīrti appear to adopt an empirical standpoint in regards  to the status of external objects, their analysis of perceptual  aspects reflects an internalist epistemology that denies the ultimate reality of external objects (see PV III, 301–319;  333–341; and Dunne 2004, 84–91). Dharmakīrti's  position regarding the reality of external objects is highly  ambivalent depending on whether he operates from a Sautrāntika  perspective, which admits the existence of external objects, or from  a Yogācāra perspective, which denies their existence (for  detailed discussions of this aspect of Dharmakīrti's thought see Dreyfus 1998, 83, 99; Dunne 2004, 53 and passim). 