All Abhidharma schools also developed complex theories of causation and conditionality to explain how dharmas interacted with each other. Another major philosophical project of the Abhidharma schools was the explanation of perception. Some schools such as the Sarvastivadins explained perception as a type of phenomenalist realism while others such as the Sautrantikas preferred representationalism and held that we only perceive objects indirectly. The major argument used for this view by the Sautrāntikas was the "time lag argument." According to Mark Siderits: "The basic idea behind the argument is that since there is always a tiny gap between when the sense comes in contact with the external object and when there is sensory awareness, what we are aware of can't be the external object that the senses were in contact with, since it no longer exists." This is related to the theory of extreme momentariness.