Plato’s Theory of Ideas or Forms

Plato thinks that there is the goodness itself in the world of Forms.

The theory of Forms (or theory of Ideas) typically refers to the belief that the material world as it seems to us is not the real world, but only an “image” or “copy” of the real world.

Plato applies this concept to all of material things.

According to Plato, there may be a tree itself in somewhere.

Trees that we can see in our lives share the property of the Form of the tree itself.

The reason why trees are trees is that they participate in the Form of the tree itself.

The reason why other things are not trees is that they don’t participate in the Form of the tree itself.


Plato’s “World of Forms”

Not only exists the world that we live our lives but also the world of Forms (or Ideas) exists.

This is what Plato actually wanted to say.

The world of Forms is the true and real world. The world that we live is the world of shadows.

The world of Forms is the original world. The world that we live is the model world.

Therefore according to Plato, we must not cling to the world that we live now.

We must seek the world of Forms.

This is the way that we do philosophy.

Zeno of Elea

Zeno of Elea (c. 490 – c. 430 BC) was a pre-Socratic Greek philosopher of Magna Graecia and a member of the Eleatic School founded by Parmenides. Aristotle called him the inventor of the dialectic.

Zeno is also regarded as the first philosopher who dealt with the earliest attestable accounts of mathematical infinity.

Zeno’s Dichotomy paradox

Suppose Homer wishes to walk to the end of a path. Before he can get there, he must get halfway there. Before he can get halfway there, he must get a quarter of the way there. Before traveling a quarter, he must travel one-eighth; before an eighth, one-sixteenth; and so on.

This description requires one to complete an infinite number of tasks, which Zeno maintains is an impossibility.


Epicurus (341–270 BC) was an ancient Greek philosopher who founded a school of philosophy now called Epicureanism.

Epicurus is a key figure in the development of science and scientific methodology because of his insistence that nothing should be believed, except that which was tested through direct observation and logical deduction. Like Democritus, he was an atomist, believing that the fundamental constituents of the world were indivisible little bits of matter (atoms; Greek: ἄτομος atom os, “indivisible”) flying through empty space (Greek: κενόν kenon). Everything that occurs is the result of the atoms colliding, rebounding, and becoming entangled with one another.

For Epicurus, the purpose of philosophy was to attain the happy, tranquil life, characterized by ataraxia—peace and freedom from fear—and aponia—the absence of pain—and by living a self-sufficient life surrounded by friends.

According to Epicurus, death is the end of both the body and the soul and therefore should not be feared. He also taught that the gods neither reward nor punish humans; that the universe is infinite and eternal.

Epicurus’ philosophy is based on the theory that all good and bad derive from the sensations of what he defined as pleasure and pain: What is good is what is pleasurable, and what is bad is what is painful. His teachings were more about striving for an absence of pain and suffering, both physical and mental, and a state of satiation and tranquillity that was free of the fear of death and the retribution of the gods. Epicurus argued that when we do not suffer pain, we are no longer in need of pleasure, and we enter a state of ataraxia, “tranquillity of soul” or “imperturbability”.

Epicurus distinguishes between two different types of pleasure: “moving” pleasures and “static” pleasures. “Moving” pleasures occur when one is in the process of satisfying a desire and involve an active titillation of the senses. After one’s desires have been satisfied, (e.g., when one is full after eating), the state of satiety is a “static” pleasure.

When a man dies, he does not feel the pain of death because he no longer is and therefore feels nothing. Therefore, as Epicurus famously said, “death is nothing to us.” When we exist, death is not; and when death exists, we are not.



Protagoras ( c. 490 – c. 420 BC) was a pre-Socratic Greek philosopher and is numbered as one of the sophists by Plato.

Protagoras is credited with the philosophy of relativism. Protagoras meant that each individual is the measure of how things are perceived by that individual. According to the philosophy of Protagoras, there is no absolute evaluation of the nature of a temperature because the evaluation will be relative to who is perceiving it.

The famous sophist Protagoras took on a pupil, Euathlus, on the understanding that the student pay Protagoras for his instruction after he wins his first court case.

Protagoras argued that if he won the case he would be paid his money. If Euathlus won the case, Protagoras would still be paid according to the original contract, because Euathlus would have won his first case.

Euathlus, however, claimed that if he won, then by the court’s decision he would not have to pay Protagoras. If, on the other hand, Protagoras won, then Euathlus would still not have won a case and would therefore not be obliged to pay.


Democritus (c.460 — c.370 BC) was an Ancient Greek pre-Socratic philosopher primarily remembered today for his formulation of an atomic theory of the universe. Many consider Democritus to be the “father of modern science”. None of his writings have survived; only fragments are known from his vast body of work.

The theory of Democritus held that everything is composed of “atoms”, which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible, and have always been and always will be in motion; that there is an infinite number of atoms and of kinds of atoms, which differ in shape and size.

The atomists agreed that motion required a void, but simply ignored the argument of Parmenides on the grounds that motion was an observable fact. Therefore, they asserted, there must be a void.

Democritus held that originally the universe was composed of nothing but tiny atoms churning in chaos, until they collided together to form larger units—including the earth and everything on it. He surmised that there are many worlds, some growing, some decaying; some with no sun or moon, some with several. He held that every world has a beginning and an end and that a world could be destroyed by collision with another world.


Empedocles (c. 490 – c. 430 BC) was a Greek pre-Socratic philosopher.

Empedocles established four ultimate elements which make all the structures in the world—fire, air, water, earth— in other words, the several states of matter are represented, being energies, gasses, liquids, and solids. Empedocles called these four elements “roots“. According to the different proportions in which these four indestructible and unchangeable elements are combined with each other the difference of the structure is produced.

The four elements are both eternally brought into union and parted from one another by two divine powers, Love and Strife. Love is responsible for the attraction of different forms of matter, and Strife is the cause of their separation.

If the four elements make up the universe, then Love and Strife explain their variation and harmony. Love and Strife are attractive and repulsive forces, respectively, which are plainly observable in human behavior, but also pervade the universe.

Like Pythagoras, Empedocles believed in the transmigration of the soul, that souls can be reincarnated between humans, animals and even plants. Empedocles was a vegetarian and advocated vegetarianism, since the bodies of animals are the dwelling places of punished souls.

Parmenides of Elea

Parmenides of Elea (c. 515 – 450 BC) was a pre-Socratic Greek philosopher from Elea. He was the founder of the Eleatic school of philosophy.

For him and his pupils, the phenomena of movement and change are simply appearances of a changeless, eternal reality. This interpretation could settle because of various wrong translations of the fragments.

Reality is one ‘thing’ (Monism) It never moves or changes. It has and will remain the same for eternity.

Reality is one ‘thing’ (Monism) It never moves or changes. It has and will remain the same for eternity. “It needs must be that what can be thought and spoken of is; for it is possible for it to be, and it is not possible for, what is nothing to be. ” It is impossible to form a concept of “nothingness” or non being.

Heraclitus of Ephesus

Heraclitus of Ephesus (c. 535 – c. 475 BC) was a pre-Socratic Greek philosopher. Little is known about his early life and education, but he regarded himself as self-taught and a pioneer of wisdom.

Ever-newer waters flow on those who step into the same rivers. Everything changes and nothing remains still … and … you cannot step twice into the same stream. 

Heraclitus considered fire as the most fundamental element. He believed fire gave rise to the other elements and thus to all things. He regarded the soul as being a mixture of fire and water, with fire being the noble part of the soul, and water the ignoble part. A soul should therefore aim toward becoming more full of fire and less full of water: a “dry” soul was best. According to Heraclitus, worldly pleasures made the soul “moist”, and he considered mastering one’s worldly desires to be a noble pursuit which purified the soul’s fire.

Pythagoras of Samos

Pythagoras of Samos (c. 570 – c. 495 BC) was an Ionian Greek philosopher and the eponymous founder of the Pythagoreanism movement. His political and religious teachings influenced the philosophies of Plato, Aristotle, and, through them, Western philosophy.

It was said that he was the first man to call himself a philosopher (“lover of wisdom”)

One of the best known mathematical formulas is Pythagorean Theorem, which provides us with the relationship between the sides in a right triangle. It states that the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides.

His teaching most securely identified with Pythagoras is metempsychosis, or the “transmigration of souls”, which holds that every soul is immortal and, upon death, enters into a new body. Pythagoras may have claimed to possess the ability to recall his former incarnations.

Another belief attributed to Pythagoras was that of the “harmony of the spheres”, which maintained that the planets and stars move according to mathematical equations, which correspond to musical notes and thus produce an inaudible symphony. The Pythagoreans believed that music was a purification for the soul, just as medicine was a purification for the body.