Like Jameson's characterization of postmodernism in terms of modes of production and technologies, the second facet, or definition, of postmodernism comes more from history and sociology than from literature or art history. The basic ideas of the Enlightenment are roughly the same as the basic ideas of humanism. I'll add a few things to her list. There is a stable, coherent, knowable self. This self knows itself and the world through reason, or rationality, posited as the highest form of mental functioning, and the only objective form.
Freedom consists of obedience to the laws that conform to the knowledge discovered by reason.
Science thus stands as the paradigm for any and all socially useful forms of knowledge. Language, or the mode of expression used in producing and disseminating knowledge, must be rational also. These are some of the fundamental premises of humanism, or of modernism. Lyotard argues that all aspects of modern societies, including science as the primary form of knowledge, depend on these grand narratives. Postmodernism then is the critique of grand narratives, the awareness that such narratives serve to mask the contradictions and instabilities that are inherent in any social organization or practice.