actuality non actuality relative modalities possibility impossibility necessity relative modalities ' 'relative'' sense depend absolute modalities something possible impossible necessary something else actual hartmann asserts reality made four levels ''inanimate'' ''biological'' ''psychological'' ' 'spiritual'' form hierarchy ref name poli ref name hartmann carnap rudolf carnap proposed truth value ontological statements existence entities depends linguistic framework statements made internal framework ref name hofweber ref name dahlstrom often
Humanities Philosophy Metaphysics Ontology