THE PRE-SOCRATICS IN A NUTSHELL

WHY GREECE?
In looking at the beginnings of philosophical thought, there is only one ancient culture who approached the subject in any meaningful and lasting way. Other cultures had forms of philosophy (as any human must have to live), but Greece, starting in the 6th Century B.C., was the first to attempt a systematic, rational pursuit of the subject. But why Greece? Greece had a unique culture compared to the rest of the ancient world. In virtually every other ancient culture, mysticism reigned. The entire workings of the world were considered to be at the whims of the Gods. Most believed that nothing in this world was intelligible, and that true reality existed in another dimension. Life was regarded as miserable, and this world viewed as evil. The Egyptian pyramids convey this philosophy explicitly, as they are monuments to death, not life. Imagine looking at your life like being trapped in a prison cell. This was the predominant ancient view. People generally don't spend their time studying the nature of the prison bars, they yearn to get out! Such was the viewpoint of the ancient world, and consequently there was little motivation or reason to start the study of philosophy. Not to mention, any who tried would probably get their heads chopped off by whatever primitive government ruled over them!
In looking at the beginnings of philosophical thought, there is only one ancient culture who approached the subject in any meaningful and lasting way. Other cultures had forms of philosophy (as any human must have to live), but Greece, starting in the 6th Century B.C., was the first to attempt a systematic, rational pursuit of the subject. But why Greece? Greece had a unique culture compared to the rest of the ancient world. In virtually every other ancient culture, mysticism reigned. The entire workings of the world were considered to be at the whims of the Gods. Most believed that nothing in this world was intelligible, and that true reality existed in another dimension. Life was regarded as miserable, and this world viewed as evil. The Egyptian pyramids convey this philosophy explicitly, as they are monuments to death, not life. Imagine looking at your life like being trapped in a prison cell. This was the predominant ancient view. People generally don't spend their time studying the nature of the prison bars, they yearn to get out! Such was the viewpoint of the ancient world, and consequently there was little motivation or reason to start the study of philosophy. Not to mention, any who tried would probably get their heads chopped off by whatever primitive government ruled over them!

Greece was different. They had a relatively large amount of political freedom compared to the rest of the world, with the exception of places like Sparta, which was essentially a military dictatorship. Consequently, Sparta didn't contribute anything meaningful to philosophical thought! While the Greeks were certainly mystical and religious on some level, their religion was unique. Their Gods were thought to have little bearing on the day to day workings of the world, and generally minded their own Godly business. They weren't omniscient, and they didn't create the world (Greeks considered the universe to always have existed). Therefore, they were liberated, in an intellectual sense, to explore and learn about the world around them. Greeks were very pro this life, and considered the world to be a good place. Consequently, as a culture they loved the pursuit of knowledge. As they began to originate the study of philosophy, two things stuck out for them; change and multiplicity. The philosophical Greeks were quite impressed by both the constant nature of change in the world, and the great many things they saw in the world. It was this interest that began the study of philosophy, and the Greek contribution to the subject is unparalleled to this day. While we like to think of ourselves as far removed from ancient cultures, it's amazing the legacy that these thinkers had on schools of thought we currently hold, and to Western Civilization as a whole.

THALES
Thales of Miletus is often considered the "Father of Philosophy", and for good reason. He attempted to explain the world of change and multiplicity without reference to mythology or the supernatural. In his attempt to explain the world of multiplicity, he looked for "the one in the many", and tried to explain change by looking for the "permanent in the changing". Out of all the stuff in the world, he thought there was one, fundamental "world stuff". This became known as "monism", the idea that everything is made up of one substance, or one stuff. Thales thought this one substance was water. At the time this wasn't so crazy, as they saw water all around them, noticed that water vanished into air if you left it out (evaporation wasn't yet understood), and saw when you dug deep into the earth, water appeared. While Thales and his followers ended up being completely wrong, it was his approach that was of profound importance. He established a naturalistic approach to the world, and laid the foundation for future scientific and philosophical progress. The "one in the many" idea was, and is, central to the advancement in science. Chemistry began to narrow down the millions of substances into fewer, simpler ones. This eventually gave us the Periodic Table, and then narrowed those elements down to just a few subatomic particles. Physics is still looking for the Unified theory, or "Theory of Everything" that will explain all the physical aspects of the universe. This attempt to simplify and understand the many things making up the world can be traced back to Thales.
Thales of Miletus is often considered the "Father of Philosophy", and for good reason. He attempted to explain the world of change and multiplicity without reference to mythology or the supernatural. In his attempt to explain the world of multiplicity, he looked for "the one in the many", and tried to explain change by looking for the "permanent in the changing". Out of all the stuff in the world, he thought there was one, fundamental "world stuff". This became known as "monism", the idea that everything is made up of one substance, or one stuff. Thales thought this one substance was water. At the time this wasn't so crazy, as they saw water all around them, noticed that water vanished into air if you left it out (evaporation wasn't yet understood), and saw when you dug deep into the earth, water appeared. While Thales and his followers ended up being completely wrong, it was his approach that was of profound importance. He established a naturalistic approach to the world, and laid the foundation for future scientific and philosophical progress. The "one in the many" idea was, and is, central to the advancement in science. Chemistry began to narrow down the millions of substances into fewer, simpler ones. This eventually gave us the Periodic Table, and then narrowed those elements down to just a few subatomic particles. Physics is still looking for the Unified theory, or "Theory of Everything" that will explain all the physical aspects of the universe. This attempt to simplify and understand the many things making up the world can be traced back to Thales.
HERACLITUS
The first villain of philosophy! Heraclitus of Ephesus wasn't all bad, but unfortunately it was mostly his harmful ideas that became influential. These ideas called into question the laws of identity and non-contradiction before they were even known! Remember that the Greeks were fascinated by change and multiplicity. Essentially, Heraclitus proclaimed that change itself is a contradiction, and thus incompatible with logic. His famous saying was "everything flows and nothing abides". So how did he come to this conclusion?
The first villain of philosophy! Heraclitus of Ephesus wasn't all bad, but unfortunately it was mostly his harmful ideas that became influential. These ideas called into question the laws of identity and non-contradiction before they were even known! Remember that the Greeks were fascinated by change and multiplicity. Essentially, Heraclitus proclaimed that change itself is a contradiction, and thus incompatible with logic. His famous saying was "everything flows and nothing abides". So how did he come to this conclusion?

Look at some examples of change around us. If you light a match, for instance, you start with a light-colored matchstick with a bright tip, but end with a black, charred remnant. It's still the same match, it's not like someone switched it, but yet it's not the same. It's a contradiction. Similarly, we were all babies once, but are now grown up. We're the same person, but yet we're not the same. Heraclitus famously noted that you could never step in the same river twice. After all, at every moment, it's changing. The banks are being eroded, water is changing levels, clarity, temperature, etc. From here, he makes an important (yet destructive) philosophical leap. There is no river. After all, what river are you referring to? The river this moment is different from the next moment, so wouldn't it be nonsensical to call it the "X River"? There is no such thing as that river, as the moment you call it "X" it has changed. But he doesn't stop there! Worse still, for Heraclitus there is no person. After all, we are changing just like the river. At every moment our bodies are changing and it's never identical. Modern science would even tell us that every 7 years all the cells in the body are replaced. If you identified a person, which one are you referring to, the one at this moment or that moment?
From this, Heraclitus concluded that everything in the world is change, as nothing remains the same. "Everything flows, and nothing abides". The world is made up of change, but not made up of entities, or actual things. After all, entities are constantly changing, so it would be nonsensical to refer to them as existing, right? What we view as things is just an illusion. Thus, Heraclitus is the first philosopher to regard our senses as invalid (though unfortunately not the last). To him, true reality is different than what we perceive. Even if you don't have the proper philosophical ammunition to refute Heraclitus, you might be thinking to yourself "this is complete lunacy, thankfully no one in the modern era would say such nonsense". Unfortunately, you'd only be half right.
"The law of contradiction is afflicted with falsity. It says nothing can both be and not be. But anything that can change defies it- it can both be and not be with the utmost ease."
-F.C.S. Schiller (a prominent American philosopher and professor at USC during the early 1900's.)
"...life consists before all just in this; that a living creature is at each moment itself, and yet something else. Life is therefore also a contradiction, present in processes, continually occurring and solving itself. And as soon as the contradiction ceases, life also ceases and death steps in."
-Friedrich Engels (the sidekick to Karl Marx)
Unfortunately, the ideas of Heraclitus continued long after his death, and even popular contemporary phrases like "there are no absolutes" and "everything is relative" have their roots in him. If you have a feeling Heraclitus and his acolytes are wrong, but can't explain why, fear not. Thankfully, you live post Aristotle, and need only to learn how he properly refuted the Heraclitians of the world.
From this, Heraclitus concluded that everything in the world is change, as nothing remains the same. "Everything flows, and nothing abides". The world is made up of change, but not made up of entities, or actual things. After all, entities are constantly changing, so it would be nonsensical to refer to them as existing, right? What we view as things is just an illusion. Thus, Heraclitus is the first philosopher to regard our senses as invalid (though unfortunately not the last). To him, true reality is different than what we perceive. Even if you don't have the proper philosophical ammunition to refute Heraclitus, you might be thinking to yourself "this is complete lunacy, thankfully no one in the modern era would say such nonsense". Unfortunately, you'd only be half right.
"The law of contradiction is afflicted with falsity. It says nothing can both be and not be. But anything that can change defies it- it can both be and not be with the utmost ease."
-F.C.S. Schiller (a prominent American philosopher and professor at USC during the early 1900's.)
"...life consists before all just in this; that a living creature is at each moment itself, and yet something else. Life is therefore also a contradiction, present in processes, continually occurring and solving itself. And as soon as the contradiction ceases, life also ceases and death steps in."
-Friedrich Engels (the sidekick to Karl Marx)
Unfortunately, the ideas of Heraclitus continued long after his death, and even popular contemporary phrases like "there are no absolutes" and "everything is relative" have their roots in him. If you have a feeling Heraclitus and his acolytes are wrong, but can't explain why, fear not. Thankfully, you live post Aristotle, and need only to learn how he properly refuted the Heraclitians of the world.

PARMENIDES
Parmenides of Elea essentially took the opposite view of Heraclitus. He knew that the ideas of Heraclitus were complete nonsense, but unfortunately came up with the opposite nonsense to combat it. He started on the right track by his famous saying, "What is is, what is not is not. What is not cannot be or be thought about." This was the earliest (though incomplete) form of "existence exists" and the 3 axioms, which later became the basis of Aristotle's metaphysics. From this statement, Parmenides drew some deductions, which start out good but quickly deteriorate.
Parmenides conclusion becomes there is no multiplicity, as there can be no spaces in existence. The world is just a densely packed "stuff" with no spaces. He calls this stuff "The One", and determines true reality is this monolithic blob of world stuff. Again, like Heraclitus, he comes to the conclusion that our senses are invalid, because we think we perceive change, but true reality is different. In examining Parmenides and Heraclitus, their differences are just as interesting (and important) as their similarities in figuring out the sources of their potential errors.
Differences:
Heraclitus viewed change as obviously true, therefore down with logic since contradictions can't exist logically.
Parmenides viewed logic as obviously true, therefore change couldn't exist since it's a contradiction.
Similarities:
Both held that change is a contradiction. Since this was their basis, they reached the inevitable conclusions on each side of that faulty coin. For philosophy to advance, this error had to be cleared up, which wasn't properly done until Aristotle.
Parmenides of Elea essentially took the opposite view of Heraclitus. He knew that the ideas of Heraclitus were complete nonsense, but unfortunately came up with the opposite nonsense to combat it. He started on the right track by his famous saying, "What is is, what is not is not. What is not cannot be or be thought about." This was the earliest (though incomplete) form of "existence exists" and the 3 axioms, which later became the basis of Aristotle's metaphysics. From this statement, Parmenides drew some deductions, which start out good but quickly deteriorate.
- The universe is uncreated
- The universe is eternal and indestructible
- There can be no empty space or vacuum, everything is solidly packed. (Parmenides considered a vacuum to be "what is not", thus couldn't exist.
- Change is impossible. (Can't go from what is to what is not, therefore the world is completely motionless. Think of a seed and a flower. A seed is, the future flower is not. Therefore, the seed can never become a flower.)
Parmenides conclusion becomes there is no multiplicity, as there can be no spaces in existence. The world is just a densely packed "stuff" with no spaces. He calls this stuff "The One", and determines true reality is this monolithic blob of world stuff. Again, like Heraclitus, he comes to the conclusion that our senses are invalid, because we think we perceive change, but true reality is different. In examining Parmenides and Heraclitus, their differences are just as interesting (and important) as their similarities in figuring out the sources of their potential errors.
Differences:
Heraclitus viewed change as obviously true, therefore down with logic since contradictions can't exist logically.
Parmenides viewed logic as obviously true, therefore change couldn't exist since it's a contradiction.
Similarities:
Both held that change is a contradiction. Since this was their basis, they reached the inevitable conclusions on each side of that faulty coin. For philosophy to advance, this error had to be cleared up, which wasn't properly done until Aristotle.

PYTHAGORAS
Because historical records are shaky, it's hard to know exactly what Pythagoras the man did vs. the Pythagorean school that was his legacy. The Pythagoreans could be thought of as the hippies of Ancient Greece, except that they were also really good at math. They essentially lived like a hippie commune, where private property was not recognized. While their beliefs and way of life were never more than on the fringe of Greek culture, they became extremely influential in a few areas, particularly in influencing Plato, and eventually Christianity, with some key ideas. The Pythagoreans practiced the Orphic Religion, which was a profoundly mystical and primitive mystery religion rooted in the Eastern tradition. The teachings and rituals were truly bizarre (you can read about them here if you're interested), and prevented the school from ever achieving lasting philosophic progress.
On the positive side, the Pythagoreans contributed much to the area of mathematics. While other ancient cultures also made mathematical discoveries, the Pythagoreans were first in developing formal proofs, and made serious inroads in the subject. Most of us remember the Pythagorean Theorem from geometry class, for example. The school also made important discoveries into how mathematics relate to other areas like music, astronomy and medicine. They discovered how mathematical ratios affect musical harmony, and to this day we describe music theory in this same way (like octaves, fifths, etc.). Their dedication to viewing mathematics as influencing all aspects of our world was long lasting on future academics, even millennia later into the Renaissance with Kepler, who searched long and hard for mathematical relationships regarding planetary motion largely because of his Pythagorean belief that mathematics governed the heavens.
On the more negative side is the Pythagorean contribution to philosophy. While Heraclitus held the world is only change, and Parmenides held the world was unchanging and made up of "The One", the Pythagoreans held the world was made up of numbers. Not just that numbers explained the world, but that they literally were the world. They held that there were two worlds; the world we perceive, which is changing, and the world of numbers, which is static and the "true reality". This true world, they said, can only be grasped through logic. The school confused numbers with physical entities themselves, and basically incorporated mathematics into their highly mystical Orphic religion, which created some truly bizarre results. For example, love was 8 (because the octave is harmony), man was 250 and plants were 360 (don't ask!). The pure nonsense of this philosophy should speak for itself, but in it's own way it was an answer to Heraclitus and Parmenides. To Heraclitus they could say, "you're right, everything is change and is flowing in this world". To Parmenides they also agreed and could say, "you're right, true reality is unchanging, it's the world of numbers".
Because historical records are shaky, it's hard to know exactly what Pythagoras the man did vs. the Pythagorean school that was his legacy. The Pythagoreans could be thought of as the hippies of Ancient Greece, except that they were also really good at math. They essentially lived like a hippie commune, where private property was not recognized. While their beliefs and way of life were never more than on the fringe of Greek culture, they became extremely influential in a few areas, particularly in influencing Plato, and eventually Christianity, with some key ideas. The Pythagoreans practiced the Orphic Religion, which was a profoundly mystical and primitive mystery religion rooted in the Eastern tradition. The teachings and rituals were truly bizarre (you can read about them here if you're interested), and prevented the school from ever achieving lasting philosophic progress.
On the positive side, the Pythagoreans contributed much to the area of mathematics. While other ancient cultures also made mathematical discoveries, the Pythagoreans were first in developing formal proofs, and made serious inroads in the subject. Most of us remember the Pythagorean Theorem from geometry class, for example. The school also made important discoveries into how mathematics relate to other areas like music, astronomy and medicine. They discovered how mathematical ratios affect musical harmony, and to this day we describe music theory in this same way (like octaves, fifths, etc.). Their dedication to viewing mathematics as influencing all aspects of our world was long lasting on future academics, even millennia later into the Renaissance with Kepler, who searched long and hard for mathematical relationships regarding planetary motion largely because of his Pythagorean belief that mathematics governed the heavens.
On the more negative side is the Pythagorean contribution to philosophy. While Heraclitus held the world is only change, and Parmenides held the world was unchanging and made up of "The One", the Pythagoreans held the world was made up of numbers. Not just that numbers explained the world, but that they literally were the world. They held that there were two worlds; the world we perceive, which is changing, and the world of numbers, which is static and the "true reality". This true world, they said, can only be grasped through logic. The school confused numbers with physical entities themselves, and basically incorporated mathematics into their highly mystical Orphic religion, which created some truly bizarre results. For example, love was 8 (because the octave is harmony), man was 250 and plants were 360 (don't ask!). The pure nonsense of this philosophy should speak for itself, but in it's own way it was an answer to Heraclitus and Parmenides. To Heraclitus they could say, "you're right, everything is change and is flowing in this world". To Parmenides they also agreed and could say, "you're right, true reality is unchanging, it's the world of numbers".

While
their bizarre number religion never took off, parts of their
philosophy were highly influential on Plato, particularly the idea that
there were two "worlds", and their formulation of the mind-body
dichotomy. The Pythagorean ethical goal was to escape the human body
and enter into their world of numbers, which brought about an inherent
conflict. The body and the mind ultimately had different goals (in their view), and thus the mind-body conflict was born. Plato ended up
formulating this idea in a much more sophisticated way, which was
eventually picked up by Christianity, and has been a dominant theme in
Western Civilization ever sense. The popular view among many to this
day is that knowledge is ideally an end in itself, and seeking knowledge
for the sake of knowledge is the most noble endeavor. This nobility
is corrupted if the knowledge is meant to further one's life, or (god
forbid!) to make money. For example, a liberal arts degree might be seen as
more lofty and noble than a trade school education, where the knowledge
is directly tied to a profession. This mindset has its roots in the
Pythagorean school, which originated with the belief that there is a separate, more noble, more "real" world we can tap into.
CONCLUSION
While the pre-Socratic philosophers came up with some faulty, and outright harmful ideas, it's invaluable to study them and learn their strengths and weaknesses. They were literally starting the field from scratch, and even the worst of them were miles ahead of other ancient cultures who were more worried about death and appeasing the Gods than figuring out the world around them. As bad as some of the pre-Socratics were, things get worse, believe it or not, before they get better. As we'll see, the Atomists and the Sophists proceed to almost bring philosophy to its knees in a disastrous dead end, before the intellectual giants of philosophy finally appear to save it.
While the pre-Socratic philosophers came up with some faulty, and outright harmful ideas, it's invaluable to study them and learn their strengths and weaknesses. They were literally starting the field from scratch, and even the worst of them were miles ahead of other ancient cultures who were more worried about death and appeasing the Gods than figuring out the world around them. As bad as some of the pre-Socratics were, things get worse, believe it or not, before they get better. As we'll see, the Atomists and the Sophists proceed to almost bring philosophy to its knees in a disastrous dead end, before the intellectual giants of philosophy finally appear to save it.