APA (7th ed.) Citation

Popper, K. R., & Hansen, T. E. (2009). The two fundamental problems of the theory of knowledge. Routledge.

Chicago Style (17th ed.) Citation

Popper, Karl R., and Troels Eggers Hansen. The Two Fundamental Problems of the Theory of Knowledge. London ; New York: Routledge, 2009.

MLA (9th ed.) Citation

Popper, Karl R., and Troels Eggers Hansen. The Two Fundamental Problems of the Theory of Knowledge. Routledge, 2009.