In the semiotic theories of Jakob von Uexküll and Thomas A. Sebeok, umwelt (plural: umwelten; from the German meaning "environment" or "surroundings") is the "biological foundations that lie at the very epicenter of the study of both communication and signification in the human [and non-human] animal". The term is usually translated as "self-centered world". Uexküll theorised that organisms can have different umwelten, even though they share the same environment. The subject of umwelt and Uexküll's work is described by Dorion Sagan in an introduction to a collection of translations.