 Dharmakīrti adds several components to the thought of Dignāga on these issues concerning valid knowledge. In defining perception as a source or means of valid knowledge, Dharmakīrti describes it as that cognition that is free from conceptuality and non-erroneous. The inclusion of the qualification of it being non-erroneous eliminates erroneous non-conceptual cognitions, such as mistakenly seeing two moons, from the category of valid knowledge. Dharmakīrti further distinguishes two ways of apprehending objects: through perceptions which apprehend real objects positively, as they are, through the mediation of images (ākāra) which perfectly reflect them, and through concepts which get at their objects negatively, through the elimination of their opposites. Dharmakīrti’s insight is found in the connection he draws between reality and the conceptual realm of universals. For Dharmakīrti, experience and representations are the bridge. We construct concepts out of our experiences and representations of functional resemblances of real entities. In other words, we can validly group together representations of actually unique entities on the basis that they can be excluded from the class of things that do not perform a given function. Georges Dreyfus put it well: