A key feature of Dignāga's logic is in how he treats generalities versus specific objects of knowledge. The Nyāya Hindu school made assertions about the existence of general principles, and in refutation Dignāga asserted that generalities were mere mental features and not truly existent. To do this he introduced the idea of "Apoha", that the way the mind recognizes is by comparing and negating known objects from the perception. In that way, the general idea or categories of objects has to do with differences from known objects, not from identification with universal truths. So one knows that a perceived chariot is a chariot not because it is in accord with a universal form of a chariot, but because it is perceived as different from things that are not chariots. This approach became an essential feature of Buddhist epistemology.