Pre-Socratic philosophy, also known as early Greek philosophy, is ancient Greek philosophy before Socrates. Pre-Socratic philosophers were mostly interested in cosmology, the beginning and the substance of the universe, but the inquiries of these early philosophers spanned the workings of the natural world as well as human society, ethics, and religion. They sought explanations based on natural law rather than the actions of gods. Their work and writing has been almost entirely lost. Knowledge of their views comes from testimonia, i.e. later authors' discussions of the work of pre-Socratics. Philosophy found fertile ground in the ancient Greek world because of the close ties with neighboring civilizations and the rise of autonomous civil entities, poleis.

Pre-Socratic philosophy began in the 6th century BC with the three Milesians: Thales, Anaximander, and Anaximenes. They all attributed the arche (a word that could take the meaning of "origin", "substance" or "principle") of the world to, respectively, water, apeiron (the unlimited), and air. Another three pre-Socratic philosophers came from nearby Ionian towns: Xenophanes, Heraclitus, and Pythagoras. Xenophanes is known for his critique of the anthropomorphism of gods. Heraclitus, who was notoriously difficult to understand, is known for his maxim on impermanence, ta panta rhei, and for attributing fire to be the arche of the world. Pythagoras created a cult-like following that advocated that the universe was made up of numbers. The Eleatic school (Parmenides, Zeno of Elea, and Melissus) followed in the 5th century BC. Parmenides claimed that only one thing exists and nothing can change. Zeno and Melissus mainly defended Parmenides' opinion. Anaxagoras and Empedocles offered a pluralistic account of how the universe was created. Leucippus and Democritus are known for their atomism, and their views that only void and matter exist. The Sophists advanced philosophical relativism. The Pre-Socratics have had significant impact on several concepts of Western philosophy, such as naturalism and rationalism, and paved the way for scientific methodology.

Pre-Socratic is a term adopted in the 19th century to refer to this group of philosophers. It was first used by the German philosopher J.A. Eberhard as vorsokratische Philosophie in the late 18th century. In earlier literature they were referred to as physikoi ("physicists", after physis, "nature"), and their activity, as physiologoi (physical or natural philosophers), with this usage arising with Aristotle to differentiate them from theologoi (theologians) and mythologoi (storytellers and bards who conveyed Greek mythology), who attributed natural phenomena to the gods.

The term was coined to highlight a fundamental change in philosophical inquiries between the philosophers who lived before Socrates — who were interested in the structure of nature and cosmos (the universe), with the implication that the universe had order to it) — and Socrates and his successors, who were mostly interested in ethics and politics. The term comes with drawbacks, as several of the pre-Socratics were highly interested in ethics and how to live the best life. Further, the term implies that the pre-Socratics are less significant than Socrates, or even that they were merely a stage (implying teleology) to classical era philosophy. The term is also chronologically inaccurate, as the last of the pre-Socratics were contemporaries of Socrates.

According to James Warren, the distinction between the pre-Socratic philosophers and the philosophers of the classical era is demarcated not so much by Socrates, but by geography and what texts survived. The shift from the pre-Socratic to the classical periods involves a shift from philosophers being dispersed throughout the Greek-speaking world to their being concentrated in Athens. Further, starting in the classical period we have complete surviving texts, whereas in the pre-Socratic era we have only fragments. Scholar André Laks [fr] distinguishes two traditions of separating pre-Socratics from Socratics, dating back to the classical era and running through current times. The first tradition is the Socratic-Ciceronian, which uses the content of their philosophical inquires to divide the two groups: the pre-Socratics were interested in nature whereas Socrates focused on human affairs. The other tradition, the Platonic-Aristotelian, emphasizes method as the distinction between the two groups, as Socrates moved to a more epistemological approach of studying various concepts. Because of the drawbacks of the term pre-Socratic, early Greek philosophy is also used, most commonly in English language philosophical literature.

André Laks [fr] and Glenn W. Most have especially popularized this shift in describing the era as "early Greek philosophy" over "pre-Socratic philosophy" through their comprehensive, nine volume Loeb editions of Early Greek Philosophy. In their first volume, they distinguish their systematic approach from that of Hermann Diels, beginning with the choice of "early Greek philosophy" over "pre-Socratic philosophy" most notably because Socrates is contemporary and sometimes even prior to philosophers traditionally considered "pre-Socratic" (such as the Atomists).

Very few fragments of the works of the pre-Socratic philosophers have survived. The knowledge we have of the pre-Socratics derives from the accounts of later writers such as Plato, Aristotle, Plutarch, Diogenes Laërtius, Stobaeus, and Simplicius, and some early Christian theologians, especially Clement of Alexandria and Hippolytus of Rome. Many of the works are titled Peri Physeos, or On Nature, a title probably attributed later by other authors. These accounts, known as testimonia (testimonies), often come from biased writers. Consequently, it is sometimes difficult to determine the actual line of argument some pre-Socratics used in supporting their views. Adding more difficulty to their interpretation is the obscure language they used. Plato paraphrased the pre-Socratics and showed no interest in accurately representing their views. Aristotle was more accurate, but saw them under the scope of his philosophy. Theophrastus, Aristotle's successor, wrote an encyclopedic book Opinion of the Physicists that was the standard work about the pre-Socratics in ancient times. It is now lost, but Simplicius relied on it heavily in his accounts.

In 1903, the German professors H. Diels and W. Kranz published Die Fragmente der Vorsokratiker (The Fragments of the pre-Socratics), which collected all of the known fragments. Scholars now use this book to reference the fragments using a coding scheme called Diels–Kranz numbering. The first two characters of the scheme are "DK" for Diels and Kranz. Next is a number representing a specific philosopher. After that is a code regarding whether the fragment is a testimonia, coded as "A", or "B" if is a direct quote from the philosopher. Last is a number assigned to the fragment, which may include a decimal to reflect specific lines of a fragment. For example, "DK59B12.3" identifies line 3 of Anaxagoras fragment 12. A similar way of referring to quotes is the system prefixed with "LM" by André Laks and Glenn W. Most who edited Early Greek Philosophy in 2016.