Friedrich Konrad Eduard Wilhelm Ludwig Klages (German: [ˈklaːɡəs]; 10 December 1872 – 29 July 1956) was a German philosopher, psychologist, graphologist, poet, writer, and lecturer, who was a two-time nominee for the Nobel Prize in Literature. In the Germanosphere, he is considered one of the most important thinkers of the 20th century. He began his career as a research chemist according to his family's wishes, though soon returned to his passions for poetry, philosophy and classical studies. He held a post at the Ludwig-Maximilians-Universität München, where in 1905 he founded the Psychodiagnostisches Seminar; the latter was forced to close in 1914 with the outbreak of World War I. In 1915, Klages moved to neutral Switzerland, where over the following decades much of his mature philosophical works were written. Klages died in 1956.

Klages was a central figure of characterological psychology and the Lebensphilosophie school of thought. Prominent elements of his philosophy include: the opposition between life-affirming Seele and life-denying Geist; reality as the ongoing creation and interpretation of sensory images, rather than feelings; a biocentric ethics in response to modern ecological issues and militarism; an affirmation of eroticism in critique of both Christian patriarchy and the notion of the "sexual"; a theory of psychology focused on expression, including handwriting analysis; and a science of character aimed at reconciling the human ego to the divide it effectuates between living beings. Central to Klages's thought is a linguistic opposition to logocentrism, a term introduced by Klages to diagnose a fixation on language or words to the detriment of the things to which they refer. His formulation of this concept came to be of significant importance to semiotic studies of Western science and philosophy, namely within Derridean deconstruction. Klages is similarly seen as a forebear to critical theory, deep ecology, and existential phenomenology.

Klages's place in modern psychology has been likened to those of his contemporaries Sigmund Freud and Carl Jung. His philosophy was roundly attacked by Nazi leaders during the height of his career, though his proximity has since fallen to dispute. Though little of his literary output has historically been available in English, Klages's thought has exhibited sweeping influence on German developments in psychology, psychiatry, literature, and various other disciplines.

Klages was born on 10 December 1872, in Hannover, Germany, the son of Friedrich Ferdinand Louis Klages, a businessman and former military officer, and wife Marie Helene née Kolster. In 1878, his sister Helene Klages was born and the two shared a strong bond throughout their lives. In 1882, when Klages was nine years old, his mother died. The death is thought to have been the result of pneumonia. Klages had begun attending school at the Lyceum am Georgsplatz (later called the Kaiser-Wilhelm-und-Ratsgymnasium [de]) in Hannover, when his aunt, Ida Kolster, soon came to live with them to help raise the children, in keeping with the dying mother's request. Klages's early education was marked by a traditional emphasis on the classics and humanities. He quickly developed a strong interest in both prose and poetry writing, as well as in Greek and Germanic antiquity. His relationship with his father was strained by the latter's strictness and will to discipline him. Nevertheless, attempts to forbid Klages from writing poetry were unsuccessful by both his teachers and parents.

Klages developed an intense childhood friendship with classmate Theodor Lessing, with whom he shared many passionate interests. Klages fought to maintain their friendship in spite of his father's antisemitism. According to Lessing, "Ludwig's father did not view his son's fraternization with 'Juden' as acceptable." In 1891, Klages completed his Abitur-level schooling and continued to Leipzig University, where he began his studies in physics and chemistry. His father had instructed him to pursue a career in industrial chemistry. He took two semesters at Leipzig, during 1891–1892, then one semester at the Technische Hochschule Hannover (now the University of Hannover), during 1892–1893.

Klages moved to Munich in 1893, continuing his undergraduate studies at the Ludwig-Maximilians-Universität München. The same year, he joined the Chemisches Institut, a laboratory founded at the university by Adolf von Baeyer in 1875. Alongside his studies, he engaged in the cultural scene in Schwabing, the Bohemian district of Munich. In 1894, Klages met poet and sculptor Hans Busse, who had recently founded the Institut für wissenschaftliche Graphologie (Institute for Scientific Graphology). Handwriting analysis, or graphology, was at that time a more reputable discipline than now; Busse was sought on occasion to give expert testimony in court cases, and his passion for the subject drew Klages to him. Others figures who entered Klages life at this time include psychiatrist Georg Meyer, poet Stefan George, classicist Alfred Schuler, and novelist Franziska zu Reventlow.

After graduating, Klages continued his work as a research chemist and began preparing his doctoral thesis under Alfred Einhorn. Klages's writings in both prose and poetry began appearing in Blätter für die Kunst, a journal publication owned by Stefan George, who himself had eagerly recognized Klages's talent. In 1896, Klages, Meyer, and Busse founded a new graphological institution, the Deutsche Graphologische Gesellschaft (German Graphological Society). Klages's childhood friendship with Theodor Lessing came to a bitter end in 1899. Both would later write about the depth of their relationship and influence on each other—though many aspects, such as the effect race had on their friendship, remain unclear.

In 1900, Klages received his doctorate in chemistry from the Ludwig-Maximilians-Universität München; since chemistry had seven years earlier moved from the medical faculty, Klages received his qualification as a philosophy doctorate (PhD) rather than a medical doctorate (MD). Klage published his thesis Attempt at a Synthesis of Menthone in 1901.