The primary characters of The World as Will and Idea are the philosophers who influenced Schopenhauer, several of whom he acknowledges and discusses in depth.

Plato

Plato is in many ways the seminal thinker of all Western philosophy: as Bertrand Russell asserts, he defined all the major issues that would be dealt with by philosophers from his time to the present. Though Plato is not the immediate antecedent of Schopenhauer's system, he is mentioned and quoted so often in The World as Will and Idea that he must be regarded as a major influence upon it.

Immanuel Kant

Kant is considered by many as the most influential philosopher of modern times (that is, from the Renaissance to the present). Schopenhauer states that, for his own readers, Kant is the only philosopher of whom he expects them to have prior knowledge in order to understand the new philosophy he now expounds. The essential point in Kant's thinking, in this context, is the idea that human perception of the external world is a kind of idea, shaped or created by that perception, and is separate from actual reality—which Kant termed the Ding an sich (the "thing in itself"). Schopenhauer's system is based upon this concept, though his elaboration is different from Kant's. For Schopenhauer, as the title of his book indicates, the world consists of two things. First, the world consists of "idea" or, as the word Vorstellung is more accurately translated, "representation" (in other words, the way external reality is presented to us or appears to us). Second, it consists of "will," which denotes the subject that perceives the outer world—that is, our inner essence as human beings. This distinction would not have been possible for Schopenhauer without the antecedent of Kant's system, enunciated in his Critique of Pure Reason (1781).

George Berkeley

Berkeley was one of the British empiricist philosophers, along with John Locke and David Hume. His contribution to philosophy is the concept that the external world is in reality a form of ideation, an "idea" in the mind of God which we humans experience. Schopenhauer only mentions Berkeley in passing. Although Berkeley's thinking (as well as Hume's) forms an antecedent to Kant, it is more specifically Kant's system with which Schopenhauer is concerned—and of which Schopenhauer's own philosophy is an elaboration and development.

Georg Wilhelm Friedrich Hegel

In every respect, Schopenhauer disliked and disagreed with Hegel's philosophy, and he derides it explicitly in the preface to the second volume of The World as Will and Idea. Hegel's philosophy is essentially a teleological one which sees world history as an inevitable process of improvement in human society. Schopenhauer resents the fact that Hegel's thinking dominated the German academic world of his time. In contrast to Hegel, Schopenhauer was a pessimist who believed that resignation, a disavowal of the possibility of genuine happiness, constituted supreme wisdom.

Gioachino Rossini

In The World as Will and Idea, Schopenhauer puts forward a system of aesthetics in which he judges music to be the supreme art form. This is because he regards literature and the visual arts as forms that merely copy the "representation" aspect of the world, while music provides us with a copy of the "will" itself and is therefore a deeper and more direct form of expression. Though Schopenhauer mostly treats music generically and without reference to specific works, the one composer he singles out seemingly for special praise is Rossini, the most important composer of Italian opera from 1810–1830. It is perhaps surprising that Schopenhauer apparently regards him as an ideal, given that Mozart and Beethoven are generally considered the greatest composers of the period from about 1775 (the start of Mozart's maturity) to 1827 (Beethoven's death). In German-speaking countries of that time (and today), Mozart and Beethoven were held in much higher regard than the popular Rossini, especially by intellectuals such as Schopenhauer himself. One therefore wonders how extensive or sophisticated Schopenhauer's actual knowledge of music was, in spite of his view that it is the highest form of art.