The central concept in phonology is the phoneme, which is a distinctive category of sounds that all the native speakers of a language or dialect perceive as more or less the same. In chapter 1, for example, you learned that although the two [k] sounds in kicked are not identical--the first one is pronounced with more aspiration than the second--they are heard as two instances of [k] nonetheless. The same is true of the [l] sound in lip and milk: They are often articulated in slightly different ways but are still regarded as essentially the same sounds by speakers of English. Since phonemes are categories rather than actual sounds, they are not tangible thing; instead, they are abstract, theoretical types or groups that are only psychologically real. (In other words, we cannot hear phonemes, but we assume they exist because of how the sounds in languages pattern as they are used by speakers.)