In the sixth and fifth centuries BC, a number of thinkers emerged in Greece who broke away from mythological thinking to approach the world in a naturalistic and increasingly scientific way.  They had no special name for themselves, but in light of their role as pioneers of what came to be known as philosophy we can call them philosophers.  Socrates came to be seen as a watershed in early Greek philosophers, so nineteenth-century AD scholars called them Presocratic philosophers, although some of them were Socrates’ contemporaries rather than predecessors.  They were especially known as philosophers of nature (Arisotle used the term physiologoi), most of whom developed their own theories of how nature works, and they provided models of the cosmos.  

Since the earliest thinkers came from Ionia (the modern Aegean coast of Turkey), they are often referred to as part of the Ionian tradition.  Their naturalistic theories provoked a reaction from Parmenides of Elea (a Greek city-state in southern Italy), who argued that change was impossible since it presupposed that what-is comes from what-is-not, beginning the Eleatic tradition of logical criticisms of natural philosophy.  The early philosophers were said to have founded “schools” such as the Eleatic school and somewhat later the atomist school.  Certainly the leading Presocratic philosophers had students, but the notion of institutions of higher learning seems anachronistic.  The early philosophers were probably more like tutors than lecturers to large classes. Nevertheless, their writings were widely read, and since the invention of the Greek alphabet (the first genuine alphabet) a century or so before they appeared, they were pioneers of prose writing for a wide audience; unlike other guilds of experts in the ancient world, they shared their ideas with anyone who was interested.

In the mid-fifth century BC there arose a movement of thinkers who called themselves Sophists.  Rather than theorizing by themselves or with a few colleagues and students, they traveled from city to city in the Greek world, giving lectures and short-term seminars for money, making them the first professors of adult education in Greece.  Some of them taught natural philosophy as part of their curriculum, but they emphasized practical over theoretical subjects: public speaking, financial management (forerunner of economics), and political science.  For this was the time when democracies arose, offering opportunities to young men who were not of the aristocratic class to become leaders of their cities.  Aristocrats had their old-boy networks to give their sons experience in leadership; the Sophists offered practical training to all who could pay their fees, a shortcut to fame and fortune.  The best Sophists became rich and famous from their practices.  

Socrates grew up in the age of democracy and of Sophistic education, but he turned away from the competitive and sometimes amoral ethos of the Sophists, as well as from the scientific approach of the natural philosophers.