What Is Life?

This volume collects three works by the theoretical physicist Erwin Schrödinger: What is Life?, a series of lectures investigating the physical basis of living organisms; Mind and Matter, philosophical essays on consciousness and its relationship to the material world; and Autobiographical Sketches, personal reflections on the author's life and career. Together they form an interdisciplinary exploration of biology, physics, and philosophy.

In the Preface to What is Life?, Schrödinger acknowledges that modern knowledge has grown too specialized for any one mind to command, yet insists that scholars must risk writing across disciplinary boundaries. He poses the book's central question: How can the events within a living organism be accounted for by physics and chemistry? His preliminary answer is that the present inability of these sciences to explain life does not mean they never can; rather, the inability itself is explainable.

Chapter 1 argues that the laws of physics are statistical throughout, deriving their precision from the cooperation of enormous numbers of atoms. Schrödinger introduces a key distinction: the chromosome fibres in living cells constitute what he calls an "aperiodic crystal," fundamentally different from the periodic crystals physicists had studied, much as an intricate tapestry differs from repeating wallpaper. He reframes the question "Why are atoms so small?" as really asking why organisms must be so large relative to atoms, and answers that an organism sensitive to single-atom impacts could not sustain orderly function. He introduces the √n rule: The relative error of any physical law is on the order of 1/√n, where n is the number of cooperating molecules, meaning organisms need comparatively large structures to benefit from accurate laws.

Chapter 2 shows that, contrary to expectation, incredibly small groups of atoms govern heredity. An organism's developmental pattern is determined by rod-like structures called chromosomes, which exist in pairs and contain a code-script for the individual's development. Schrödinger describes mitosis (cell division in which every chromosome duplicates so each daughter cell receives a complete set) and meiosis (reductive division producing sex cells, or gametes, each carrying only a single chromosome set). He estimates the maximum size of a gene at roughly a cube 300 Ångströms (one ten-billionth of a meter) on a side, containing no more than about a million atoms, far too few to exhibit orderly behavior according to statistical physics.

Chapter 3 introduces mutations as the true raw material for natural selection, replacing Charles Darwin's reliance on small continuous variations. Schrödinger presents the Dutch botanist Hugo de Vries's discovery that offspring occasionally exhibit rare, discrete changes with no intermediate forms, and that these mutations breed perfectly true. He notes that the foundational laws of heredity were established by the Augustinian abbot Gregor Mendel through breeding experiments with garden peas in the 1860s, rediscovered in 1900. A mutation affects only one chromosome of a pair: when the mutant version is recessive (masked by the dominant version), it influences the organism's visible characteristics only when present on both chromosomes, a condition made more likely by inbreeding. Experiments with X-rays showed that mutations increase in exact proportion to radiation dosage, proving that each mutation results from a single ionization event within a critical volume of roughly 1,000 atoms.

Chapter 4 argues that the permanence and discrete behavior of genes are inexplicable by classical physics but fully explicable by quantum theory, the branch of physics governing phenomena at atomic and subatomic scales. Schrödinger illustrates the puzzle with the Habsburg lip, a hereditary trait faithfully reproduced across centuries despite the gene's containing perhaps only a thousand atoms exposed to constant thermal agitation. The Heitler-London theory of the chemical bond, developed in 1926-27, revealed that molecules occupy discrete energy levels and cannot change configuration unless supplied with sufficient energy. The exponential dependence of molecular stability on the ratio W/kT (threshold energy to average thermal energy) shows that small changes in this ratio produce enormous differences in stability, from fractions of a second to tens of thousands of years.

Chapter 5 presents the model of the German physicist Max Delbrück: The gene is a huge molecule capable only of discontinuous rearrangements, with energy thresholds high enough to make spontaneous change rare. These rare events are mutations. The chromosome fibre is an "aperiodic solid" in which every atom plays an individual role, capable of encoding enormous informational variety within a tiny volume. Schrödinger tests the model against biological evidence, finding that the required threshold energies fall within the range known from ordinary chemistry and that the temperature dependence of mutation rates matches the exponential formula's predictions.

Chapter 6 draws what Schrödinger identifies as his primary motive for writing: Living matter, while not eluding known laws of physics, likely involves "other laws of physics" not yet discovered. The key principle is "order based on order": whereas known physical laws derive regularity from the statistical averaging of disordered atoms, the hereditary substance maintains order through its own molecular structure, safeguarded by quantum mechanics. Organisms avoid decay to thermodynamic equilibrium (maximum entropy, or complete disorder) by drawing "negative entropy" from their environment. Chapter 7 distinguishes two mechanisms for producing orderliness: the "statistical mechanism" producing "order from disorder" (the basis of thermodynamics) and the biological mechanism producing "order from order." The organism hinges upon a solid structure, the aperiodic crystal, kept in shape by quantum-mechanical forces strong enough to resist thermal disruption.

The Epilogue offers Schrödinger's reflections on determinism and free will. He presents two seemingly contradictory premises, that the body functions as a pure mechanism and that one knows by direct experience that one directs its motions, and infers that the conscious self is the entity controlling the atoms according to natural law. He connects this to the ancient Hindu Upanishadic teaching, found in sacred texts exploring the nature of the self, that the personal self equals the universal self.

Mind and Matter extends these philosophical investigations. Chapter 1 proposes that consciousness accompanies processes of learning and novelty: repeated routines fade from awareness, while deviations restore consciousness. Chapter 2 addresses whether further human evolution remains possible. Drawing on the biologist Julian Huxley's Evolution: A Modern Synthesis, Schrödinger argues that behavior powerfully influences evolution: a favorable mutation causes organisms to change behavior in ways that intensify selective pressure for further mutations in the same direction, effectively simulating the Lamarckian idea that acquired traits are inherited, without requiring it. Chapter 3 explains the "principle of objectivation," science's methodological exclusion of the knowing subject from its world picture, and concludes that the barrier between subject and object does not truly exist. Chapter 4 addresses the "arithmetical paradox" of many conscious minds in a single world, arguing that mind is by nature a singulare tantum, a singular entity. Chapter 5 examines how the idealization of time by Plato, Kant, and Einstein supports the possibility of timeless existence. Chapter 6 explores the paradox that all scientific knowledge rests on sense perception, yet the scientific picture contains no sensual qualities.

The Autobiographical Sketches recount the formative influence of Schrödinger's father, Rudolph, who engaged his son in wide-ranging conversation, and of the physicist Ludwig Boltzmann, whose ideas reached Schrödinger through the lectures of Boltzmann's successor Fritz Hasenöhrl. Schrödinger details the deprivations of post-World War I Vienna, his father's death in 1919, and his mother's death in 1921. He summarizes his career: his first Viennese years; his discovery of wave mechanics in Arosa, Switzerland, in 1925; his Berlin appointment as Max Planck's successor; his later years of roaming after Hitler's rise to power, through Oxford, Graz, and Belgium; his 17-year exile in Dublin; and his return to Vienna in 1956. He reflects on the equal importance of heredity and environment in shaping individuals and concludes with a November 1960 note stating his decision not to continue the sketches.