In the 1960s, Chomsky introduced two central ideas relevant to the construction and evaluation of grammatical theories. One was the distinction between "competence" and "performance". Chomsky noted the obvious fact that when people speak in the real world, they often make linguistic errors, such as starting a sentence and then abandoning it midway through. He argued that such errors in linguistic "performance" are irrelevant to the study of linguistic "competence", the knowledge that allows people to construct and understand grammatical sentences. Consequently, the linguist can study an idealised version of language, which greatly simplifies linguistic analysis (see the "Grammaticality" section below).