知る vs 知っている (know: perfective vs state)

The verb 知る has an asymmetry: the plain affirmative present 知る means 'come to know' (perfective), while the state of knowing requires ~ている: 知っている ('know'). Negation, however, returns to the plain form: 知らない ('don't know'), not 知っていない. This asymmetry catches many learners.