This article offers the first phenomenological clarification of the objective conditions for sensory illusions focusing on the noematic ‘idea’ and a priori ontological structure that regulate the appearances of illusory things in perception. First, I will show the necessity of eidetic and noematically oriented investigations, arguing that it is an analysis of how the essence ‘Thinghood’ functions transcendentally that leads to an a priori clarification of both illusory things and their corresponding multiplicities of appearances. Accordingly, I will consider the correlate of material perception according to both its formal and material ontological structures and focus on the a priori principle of coordination and the laws of mutability and reversibility that set the framework for an analysis a parte obiecti. I will then examine the process of typification, the psycho-physical conditionality of the lived-body, and the constitution of different levels of normality and abnormality. Within this expanded analysis, I will provide a clarification of ‘false’ sensory appearances showing how both deceptive and resilient illusory cases can be scientifically accounted for on a phenomenological basis and in accordance with natural-empirical laws.