Metaphysics by Aristotle is the single most important ancient source for reconstructing Presocratic philosophy, whose original writings largely survive only in fragments. In the Metaphysics, Aristotle offers the earliest systematic history of philosophy, explicitly analyzing thinkers such as Thales, Anaximander, Anaximenes, Heraclitus, Parmenides, Empedocles, and the early atomists. He preserves their core doctrines—especially the search for first principles (archai), explanations of nature (physis), and early conceptions of being—while situating them within a coherent philosophical development leading to later metaphysics. Because Aristotle had access to sources now lost, including early Peripatetic scholarship, his Metaphysics functions as an indispensable transmission and interpretive framework, anchoring modern knowledge of Presocratic thought and its foundational role in Western philosophy.

Physics by Aristotle is one of the most important ancient sources for understanding Presocratic philosophy, whose original works largely survive only in fragments. In the Physics, Aristotle examines early Greek attempts to explain nature (physis), motion, change, and causation, preserving doctrines from thinkers such as the Milesians, Heraclitus, Parmenides, Empedocles, and Anaxagoras. He records how Presocratic philosophers sought natural principles (archai) and rational explanations in place of myth, marking the birth of scientific and philosophical inquiry. Because Aristotle critically analyzes these early theories while drawing on sources now lost, his Physics serves as a vital transmission and interpretive framework, enabling modern scholars to reconstruct the foundations of Western natural philosophy.

On the Heavens by Aristotle is a crucial ancient source for reconstructing Presocratic philosophy, especially early Greek cosmology and natural science. In this work, Aristotle preserves and critiques theories about the structure, motion, and composition of the cosmos proposed by thinkers such as the Milesians, Pythagoreans, Xenophanes, and other early natural philosophers. He records their attempts to explain the heavens through natural principles rather than myth, including ideas about the shape of the Earth, the nature of celestial bodies, and cosmic order. Because most Presocratic writings are lost, On the Heavens functions as an essential transmission source, safeguarding early Greek cosmological thought and illuminating how Presocratic philosophy laid the foundations for later astronomy and metaphysics.

The Fragments of Xenophanes preserve the surviving thought of Xenophanes, a 6th–5th century BCE Greek philosopher and poet known for his radical critique of anthropomorphic religion. Written in elegiac and hexameter verse, the fragments challenge traditional portrayals of the gods, advancing the idea of a single, supreme, non-anthropomorphic deity and emphasizing the limits of human knowledge. Central to Pre-Socratic theology and epistemology, the Fragments of Xenophanes are essential for understanding early philosophical monotheism, skepticism, and the transition from mythic to rational religious thought in ancient Greece.

The surviving fragments of Heraclitus, written around c. 500 BCE, represent one of the most influential bodies of thought in early Western philosophy. Preserved only in quotations by later authors such as Plato, Aristotle, and the Stoics, these fragments explore core concepts including logos, constant change, and the unity of opposites, famously expressed in the idea that one cannot step into the same river twice. Heraclitus presented a vision of reality defined by perpetual flux, tension, and transformation, challenging static views of being held by earlier thinkers. His aphoristic and often enigmatic style earned him the nickname “the Obscure,” yet his ideas profoundly shaped Stoicism, Platonism, and later metaphysical and dialectical traditions. 

Parmenides’ On Nature is a foundational Pre-Socratic philosophical poem, composed in the early 5th century BCE, that profoundly shaped Western metaphysics and ontology. Written in dactylic hexameter, the work presents a revelatory journey in which a goddess instructs the thinker on the Way of Truth—asserting the unity, permanence, and unchanging nature of Being—and the Way of Opinion, which explains the deceptive world of appearances. Preserved in fragments, On Nature is essential for understanding Eleatic philosophy, the rejection of becoming, and the origins of rational metaphysical inquiry, exerting lasting influence on Plato, Aristotle, and the development of classical philosophy.

The Fragments of Empedocles preserve the surviving portions of the thought of Empedocles, a 5th-century BCE Greek philosopher and poet whose work bridged cosmology, metaphysics, religion, and science. Composed in poetic form and traditionally divided between On Nature and Purifications, the fragments articulate Empedocles’ doctrine of the four elements—earth, air, fire, and water—governed by the cosmic forces of Love and Strife. Central to Pre-Socratic philosophy and early natural science, the Fragments of Empedocles are essential for understanding pluralist metaphysics, ancient cosmology, and the transition from mythic to rational explanations of nature.

The Fragments of Anaxagoras preserve the surviving thought of Anaxagoras, a 5th-century BCE Greek philosopher who played a crucial role in the development of ancient cosmology and rational science. Known for introducing Nous (Mind) as the organizing principle of the universe, Anaxagoras argued that all things are composed of infinitely divisible seeds (homoiomeries) rather than a single primary substance. Preserved through later authors, these fragments are essential for understanding Pre-Socratic pluralism, early scientific reasoning, and the shift toward explanatory intellect in Greek philosophy, exerting lasting influence on Plato, Aristotle, and classical natural philosophy.

The Fragments of Democritus preserve the surviving writings of Democritus, a 5th-century BCE Greek thinker and a founder of ancient atomism. These fragments outline Democritus’ doctrine that all reality consists of indivisible atoms moving in the void, governed by necessity rather than divine intervention, alongside ethical reflections on cheerfulness (euthymia) and the good life. Transmitted through later philosophers and commentators, the Fragments of Democritus are essential for understanding early materialism, scientific rationalism, and the foundations of physical theory, and they profoundly influenced Epicurean philosophy and the history of natural science.