Parmenides of Elea marks a turning-point in the history of philosophy: his investigations, supported and supplemented by those of his two followers, seemed to reveal deep logical flaws in the very foundations of earlier thought. Science, it appeared, was marred by subtle but profound contradictions; and the great enterprise undertaken by the Milesians, by Xenophanes and by Heraclitus, lacked all pith and moment. The age of innocence was ended, and when science was taken up again by the fifth-century philosophers, their first and most arduous task was to defend their discipline against the arguments of Elea. If their defence was often frail and unconvincing, and if it was Plato who first fully appreciated the strength and complexity of Parmenides’ position, it remains true that Parmenides’ influence on later Presocratic thought was all-pervasive. Historically, Parmenides is a giant figure; what is more, he introduced into Presocratic thought a number of issues belonging to the very heart of philosophy.