 Two realist schools of Buddhism, the Vaibhāṣika and the Sautrāntika, also present an atomistic conception of nature. According to the former, matter is a collocation of the substratum of colour, taste, odour and touch. Atoms are the minutest units of the rūpa-skandha (collocation of material elements). As it is mentioned in the Abhidharmakośa (I. 44), ‘Atoms of the visual organ are arranged in the pupil of the eye in the shape of an ajājī flower; those of the auditory organ are arranged in the earhole in the shape of a bhūrja leaf, atoms of the olfactory organ are arranged in the form of a long pin (śalākā) inside the nostrils, those of the gustatory organ inside the mouth in the shape of the half-moon, and those of the cutaneous organ in the shape of the body.’ Atoms are thus indirectly related to observational entities. Atoms, according to them, are indivisible, imperceptible and momentary. They continually undergo phase-changes. Some Sautrāntikas hold that atoms are not particles of matter but a dynamic force or energy. According to Vasubandhu, atoms are always in an aggregate and never alone. For it has been mentioned by some that the rūpa-skandha is that which can cause obstruction and is also subject to transformation. A single atom cannot possess these properties; hence atoms are always in a cluster.