Friedrich Karl Christian Ludwig Büchner (March 29, 1824 – May 1, 1899) was a German philosopher, physiologist and physician who became one of the exponents of 19th century scientific materialism.

Büchner was born at Darmstadt, Germany, on March 29, 1824. From 1842 to 1848 he studied physics, chemistry, botany, mineralogy, philosophy and medicine at the University of Giessen, where he graduated in 1848 with a dissertation entitled Beiträge zur Hall'schen Lehre von einem excitomotorischen Nervensystem (Contributions to the Hallerian Theory of an Excitomotor Nervous System). Afterwards, he continued his studies at the University of Strasbourg, University of Würzburg (where he studied pathology with the great Rudolf Virchow) and at the University of Vienna. In 1852 he became lecturer in medicine at the University of Tübingen, where he published his great work Kraft und Stoff: Empirisch-naturphilosophische Studien (Force and Matter: Empiricophilosophical Studies, 1855). In this work, the product, according to Lange, of a fanatical enthusiasm for humanity, he sought to demonstrate the indestructibility of matter and force, and the finality of physical force. The extreme materialism of this work excited so much opposition that he was compelled to give up his post at Tübingen. He retired to Darmstadt, where he practiced as a physician and contributed regularly to pathological and physiological magazines.

He continued his philosophical work in defense of materialism, and published Natur und Geist (Nature and Soul, 1857), Aus Natur und Wissenschaft (From Nature and Science, vol. I., 1862; vol. II., 1884), Der Fortschritt in Natur und Geschichte im Lichte der Darwinschen Theorie (Progress in Nature and History in the Light of the Darwinian Theory, 1884),Tatsachen und Theorien aus dem naturwissenschaftlichen Leben der Gegenwart (Facts and Theories from the Scientific Life of Present, 1887), Fremdes und Eigenes aus dem geistligen Leben der Gegenwart (Extraneous and Self from the Spiritual Life of Present, 1890), Darwinismus und Socialismus (1894), Im Dienste der Wahrzeit (In the Service of Truth, 1899).

Ludwig Büchner's materialism was the founding ground for the freethinkers' movement in Germany. In 1881 he founded in Frankfurt the "German Freethinkers League" ("Deutsche Freidenkerbund"), where the first atheists gathered together publicly in that country.

He died at Darmstadt on May 1, 1899.

In evaluating Büchner's philosophy it must be remembered that he was primarily a physiologist, not a metaphysician. Matter and force (or energy) are infinite; the conservation of force follows from the imperishability of matter, the ultimate basis of all science. Büchner is not always clear in his theory of the relation between matter and force. At one time he refuses to explain it, but generally he assumes that all natural and spiritual forces are indwelling in matter. Just as a steam engine, he says in Kraft und Stoff (7th ed., p. 130), produces motion, so the intricate organic complex of force-bearing substance in an animal organism produces a total sum of certain effects, which, when bound together in a unity, are called by us mind, soul, thought. Here he postulates force and mind as emanating from original matter, a materialistic monism. But in other parts of his works he suggests that mind and matter are two different aspects of that which is the basis of all things, a monism which is not necessarily materialistic.

Büchner was much less concerned to establish a scientific metaphysics than to protest against the romantic idealism of his predecessors and the theological interpretations of the universe. Nature according to him is purely physical; it has no purpose, no will, no laws imposed by extraneous authority, no supernatural ethical sanction.

Modern Christian apologists consider Büchner the father of atheistic evangelism in Germany, a counterpart to Thomas Huxley.

Ludwig Büchner was the brother of Georg Büchner, a famous playwright.