 Continental philosophy is more difficult to define as a class. Somewhat ironically, the concept of continental philosophy is a creation of philosophers in the United States who focus on (or who are influenced by) the work of post-Enlightenment European thinkers, especially those in France and Germany. While the range of European figures studied by continental philosophers is too large and varied to list comprehensively here, one could say that it begins with nineteenth century theorists such as Georg Wilhelm Friedrich Hegel, Søren Kierkegaard, Karl Marx, and Friedrich Nietzsche; continues with twentieth century thinkers such as Theodor Adorno, Louis Althusser, Walter Benjamin, Simone de Beauvoir, Gilles Deleuze, Michel Foucault, Sigmund Freud, Hans-Georg Gadamer, Martin Heidegger, Edmund Husserl, Jacques Lacan, Jean-François Lyotard, Herbert Marcuse, Maurice Merleau-Ponty, and Jean-Paul Sartre; and extends into the twenty-first century with contemporary writers such as Pierre Bourdieu, Jacques Derrida, Jürgen Habermas, Julia Kristeva, and Luce Irigaray.