 The existence of matter can also not be inferred on the basis of the prima facie existence of what I perceive “by means of the sense organs of our bodies”, i.e., of what he calls “sensa” (NE §373), because it is erroneous to believe that matter as the presumed outside cause of a sensum has the same qualities as a sensum and thus has to exist (NE §365). He conjectures that if there are outside causes of sensa they must be substances which are “of a spiritual nature” (NE §371). When it comes to (c) sensa McTaggart holds that one has to distinguish between two classes of percepta, those perceived by introspection (mental states, spiritual data) and those that are given by means of sense organs (sensa). The latter do not really exist, they just lead to the illusion that they exist. This is so because of a confusion between a perception that is part of the percipient and therefore spiritual or mental in character and what is perceived, i.e., the object of a perception or the perceptum (NE §373). However, a perceptum as a sensum cannot, according to McTaggart, have parts within parts to infinity and thus cannot really exist because what exists has no simple parts (cf. 355). Having disposed of matter and sensa this way, he then discusses the ontological status of (d) spirit or spirituality. He declares that “the quality of spirituality … is the quality of having content, all of which is the content of one or more selves” (NE §381) and states that “nothing can have this quality except substances, and so nothing but substances are spiritual” and exist or are real (ibid.). A self or an I he takes to be a simple quality of a substance which is known to me to be myself by direct perception, i.e., is known by acquaintance, not by description. The distinction between knowledge by acquaintance and knowledge by description he explicitly takes up from Russell (NE §382). He then surmises that it is very likely that the I, i.e., the substance that possesses the quality of being a self, persists through time because “I perceive myself as persisting through time, or the real series which appears as a time-series” (NE §395). He also holds that selves are conscious without having to be self-conscious (NE §397) and that no experience is possible “which is not part of a self” (NE §400) though it cannot belong to more than one self (NE §401). He concludes: 