Teachers are necessarily free. The present article discusses the dual meaning of this necessity. The first meaning relates to freedom as an inevitable aspect of the actual reality in the classroom (the "is"); the second to teachers' freedom as the ideal condition, or a prerequisite for optimal teaching (the "ought"). Existentialist philosopher Jean-Paul Sartre argued that human beings are "condemned to be free" and demanded that freedom be considered an imperative value. Philosopher of education Joseph Schwab, who analysed the practical nature of teaching, concluded that teachers make choices every minute in the classroom, and demanded that their autonomy be expanded. This article identifies parallels between these two philosophical moves and suggests complementing features. Sartre adds universal depth to Schwab's portrait of teachers, proving them to be an extreme expression of the human condition. Schwab provides Sartre's analysis with concrete validation and practical suggestions for implementation. Combining these two approaches can further establish the importance of teacher autonomy and offer a unique conceptualisation of teacher agency as a partial response to the crisis of education in the postmodern era.