This is a thematic guide to the work of the PreSocratics. It provides an accessible introduction to the principles which paved the way to modern theories. The PreSocratics were not scientists in the modern sense of the word but existed long before Archimedes and his cry of "Eureka". The were the pioneers of European philosophy and science - Democritus, Heraclitus and Empedocles. The author has published translations of Plato's "Philebus", "Theaetetus", "Early Socratic Dialogues" and Xenophon's "Conversations of Socrates".