A trained philologist, Nietzsche had a thorough knowledge of Greek philosophy. He read Kant, Plato, Mill, Schopenhauer and Spir, who became his main opponents in his philosophy, and later Baruch Spinoza, whom he saw as his "precursor" in many respects but as a personification of the "ascetic ideal" in others. However, Nietzsche referred to Kant as a "moral fanatic", Plato as "boring", Mill as a "blockhead", and of Spinoza he said: "How much of personal timidity and vulnerability does this masquerade of a sickly recluse betray?" He likewise expressed contempt for British author George Eliot.
Nietzsche's philosophy, while innovative and revolutionary, was indebted to many predecessors. While at Basel, Nietzsche offered lecture courses on pre-Platonic philosophers for several years, and the text of this lecture series has been characterized as a "lost link" in the development of his thought. "In it concepts such as the will to power, the eternal return of the same, the overman, gay science, self-overcoming and so on receive rough, unnamed formulations and are linked to specific pre-Platonics, especially Heraclitus, who emerges as a pre-Platonic Nietzsche." The pre-Socratic thinker Heraclitus was known for the rejection of the concept of being as a constant and eternal principle of universe, and his embrace of "flux" and incessant change. His symbolism of the world as "child play" marked by amoral spontaneity and lack of definite rules was appreciated by Nietzsche. From his Heraclitean sympathy, Nietzsche was also a vociferous detractor of Parmenides, who opposed Heraclitus and believed all world is a single Being with no change at all.
In his Egotism in German Philosophy, Santayana claimed that Nietzsche's whole philosophy was a reaction to Schopenhauer. Santayana wrote that Nietzsche's work was "an emendation of that of Schopenhauer. The will to live would become the will to dominate; pessimism founded on reflection would become optimism founded on courage; the suspense of the will in contemplation would yield to a more biological account of intelligence and taste; finally in the place of pity and asceticism (Schopenhauer's two principles of morals) Nietzsche would set up the duty of asserting the will at all costs and being cruelly but beautifully strong. These points of difference from Schopenhauer cover the whole philosophy of Nietzsche."
Nietzsche expressed admiration for 17th-century French moralists such as La Rochefoucauld, La Bruyère and Vauvenargues, as well as for Stendhal. The organicism of Paul Bourget influenced Nietzsche, as did that of Rudolf Virchow and Alfred Espinas. Nietzsche wrote in a letter in 1867 that he was trying to improve his German style of writing with the help of Lessing, Lichtenberg and Schopenhauer. It was probably Lichtenberg (along with Paul Rée) whose aphoristic style of writing contributed to Nietzsche's own use of aphorism instead of an essay. Nietzsche early learned of Darwinism through Friedrich Albert Lange. The essays of Ralph Waldo Emerson had a profound influence on Nietzsche, who "loved Emerson from first to last", wrote "Never have I felt so much at home in a book", and called him "[the] author who has been richest in ideas in this century so far." Hippolyte Taine influenced Nietzsche's view on Rousseau and Napoleon. Notably, he also read some of the posthumous works of Charles Baudelaire, Tolstoy's My Religion, Ernest Renan's Life of Jesus and Fyodor Dostoyevsky's Demons. Nietzsche called Dostoevsky "the only psychologist from whom I have anything to learn." While Nietzsche never mentions Max Stirner, the similarities in their ideas have prompted a minority of interpreters to suggest a relationship between the two. In 1861 Nietzsche wrote an enthusiastic essay on his "favorite poet", Friedrich Hölderlin, mostly forgotten at that time. He also expressed deep appreciation for Stifter's Indian Summer, Byron's Manfred and Twain's Tom Sawyer.