According to Buddhologist Tom Tillemans, Dharmakīrti's ideas constitute a nominalist philosophy which disagrees with the Madhyamaka philosophy, by asserting that some entities are real. Dharmakīrti states that the real is only the momentarily existing particulars ("svalakṣaṇa"), and any universal ("sāmānyalakṣaṇa") is unreal and a fiction. He criticized the Nyaya theory of universals by arguing that since they have no causal efficacy, there is no rational reason to posit them. What is real must have powers ("śakti"), fitness ("yogyatā") or causal properties which is what individuates a real particular as an object of perception. Dharmakīrti writes "whatever has causal powers ("arthakriyāsamartha"), that really exists ("paramārthasat")." This theory of causal properties has been interpreted as a form of trope theory. "Svalakṣaṇa" are said to be part-less, undivided and property-less, and yet they impart a causal force which give rise to perceptual cognitions, which are direct reflections of the particulars.