In the introduction and chapter one, I define “philosophical amateurism” as practiced by musicians and, especially, composers, as something characterized by a specifically musical way of reacting to or engaging with philosophical texts. This amateurism frequently involves poetization of the philosophical material, embodied engagement with philosophical texts, and an aspirational approach to esoteric or abstract realms of thought. Chapter one looks at the implications of this idea for fin-de-siècle and early twentieth-century settings of Nietzsche’s poem “O Mensch, gib Acht,” from Also Sprach Zarathustra. I show that compositions ranging from Mahler’s Symphony no. 3 to songs and choral works by numerous professional and amateur musicians exemplify an amateur’s approach to Nietzsche in their focus on a short poem from the book and in their outreach to amateur musical participation, as well. Chapter two focuses on semi-operatic and operatic works from the early to mid-twentieth century that model an amateur engagement with philosophy by substituting the life story of major philosophers for an argumentative treatment of their texts, including Erik Satie’s Socrate, Johanna Bordewijk-Roepman’s Plato’s Dood and Julius Drossin’s Spinoza: A Life in Three Acts. In all three cases a philosopher’s person comes to be a corporeal representation of the place of their philosophical contributions in daily life, and the stories told often reflect other important aspects of the works' contexts. Chapter three, based on interviews with composers at a variety of career stages today, looks at how individuals and broader communities of composers today view the place of “composing philosophy” as it is primarily discussed in this project; while some composers find it to be a profitable practice, within certain limited frameworks, others also express distrust respecting uses of philosophy that are too overt or not well-enough integrated into the fibers of a composition; still more suggest that to engage with new music from, say, feminist or other gendered perspectives might be to leave what is typically understood to be “philosophical” material behind. Chapter four turns to the matter of “philosophical” instrumental program music, by contextualizing Leonard Bernstein’s programmatic instrumental work, the Serenade (after Plato’s “Symposium”) for Violin, String Orchestra, and Percussion within Bernstein’s reputation as an “accessible” tonalist modernist composer. It then focuses on the role of Plato’s homoerotic text on the intensely positive private reception that the piece received within Bernstein’s circle of normative-male homosexual close friends and colleagues, as well as the way that the piece’s musical style can be said to be in conversation with queer ideas about time both in the period of the piece’s composition and more recently. Chapter five offers a case study of Kate Soper’s IPSA DIXIT (Pulitzer Prize finalist in 2017), which uses large portions of Aristotle’s Poetics, Rhetoric, and Metaphysics for its libretto, focusing on how Soper’s piece, which strongly involves a woman soprano in a sometimes monodramatic role while also emphasizing intensive chamber-ensemble collaboration, offers a deep and witty critique of philosophical authority that invokes feminist relational aesthetics. Chapter six concludes the book with a discussion of what I call “the persistence of practices,” remarking on the particular upswing of pieces that involve “composing philosophy” that have just appeared in the past twenty years.