Plato wanted to solve the age-old philosophical problem with which the natural philosophers had struggled: what is eternal and immutable on the one hand and what "flows" with time on the other.

The Sophists and Socrates redirected philosophy to consideration of problems related to man and society, but they disagreed about the nature of human morality. The Sophists believed that morality varied according to human cultures and situations (moral relativism). The truth depended solely on the ability of a speaker to persuade his audience of the rightness of his position. Socrates believed in the existence of eternal and absolute truth (ideals), and he insisted that right and wrong could be determined by examining our behavior with reason. However, Socrates had not applied his conceptions of truth and the soul to the material realm in a systematic way.

As Democritus and Empedocles had done in the past,  Plato sought to unite the natural philosophers' focus on the substance and shape of reality with the idealists' interest in the eternal. He tried to grasp a "reality" that was eternal and immutable but could also explain the changes we observe with our senses.

Plato's solution to the problem was to assert that everything in the material world flows. Everything discernable in the world of the senses is subject to time and ultimately decays and passes away, even the most basic elements; however, a timeless mold or "Form" shapes everything in the material world.

A real horse is born, grows, changes and dies, but the "Idea of the Horse" is eternal. Ideas are not made from physical substances; they are spiritual and abstract, eternal and immutable. You probably cannot draw a perfect circle no matter how hard you try, but you can conceive of the idea of a circle with your reason. 

Plato's idea solved one of the problems that the natural philosophers could not. Empedocles and Democritus could assert the existence of an eternal physical substance (the four elements or atoms), but they could not come up with a theory which explained how or why these elements arranged themselves into elephants and crocodiles, not elodiles or crocophants. Democritus insisted that nature proceeded randomly. Plato insisted that nature was built according to a plan based on a limited number of forms. Behind every horse is the Form of a Horse. These Forms must exist in a reality separate from the material world. He called this realm The World of Ideas.

Plato believed that nothing in the world of the senses could last: everything physical must inevitably disintegrate and pass away. Not only will material substances, like people or buildings, disintegrate, but all human ideas will also perish. We can never have true knowledge of anything that is in a constant state of change. We can only form inexact opinions about the material world which we perceive with our limited senses. However, true knowledge of Ideas in the World of Forms can be achieved through deductive reasoning. Reason expresses eternal and universal truths. For example, mathematical truths never change. The sum of angles in a triangle must always equal 180 degrees.  

Plato believed that humans exist simultaneously in the realm of the senses and in the World of Ideas. Our body "flows" in the realm of the senses, but we each have an immortal soul which exists in the World of Ideas. Man, like the universe, has a dual nature.

For Plato, the soul is immortal. The soul existed before inhabiting our bodies, but at the moment we are born, we forget about our existence in this perfect realm. Yet, as we develop reason, vague recollections of the World of Ideas are stirred within us, and then the soul yearns to return to its true realm. From this moment on, the whole sensory world is experienced as imperfect and insignificant, and our soul longs to be freed from the chains of the body. We understand that all natural phenomena are merely shadows of eternal forms and ideas.

This myth illustrates the challenges that the philosopher must face as he learns to distinguish between shadowy images and the true ideas behind all natural phenomena. 

In The Republic, Plato described his idea of the best political system, a Utopian State. The people would not run his ideal government. Instead, he believed that philosophers should govern the state.

He likened the state to the human body: the head, the chest and the gut. Reason belongs to the head, will belongs to the chest, and appetite belongs to the gut. Reason aspires to wisdom, will aspires to courage, and appetite must be curbed by temperance. When the three parts of the body function harmoniously, we achieve virtue as individuals and harmony as a state. In Plato's Utopia, philosophers rule, soldiers protect, and laborers produce food and shelter. In a virtuous state, everyone knows his or her place. (You can compare this conception of the state to the Hindu caste system.)

Basically, Plato believed in a totalitarian state, run by an absolute ruler, the Philosopher King. Plato also believed that since education was essential to creating his Utopia, raising children should be the responsibility of the state not of parents. A ridiculous idea, huh? He also believed that women were just as capable of running society as men.

Plato believed that there could not be material forms without an ideal mold. The idea "chicken" had to come before both the chicken and the egg.

Aristotle could find no evidence to support Plato's belief in a World of Forms. He thought that the 'abstract idea' of a chicken was simply a concept that humans themselves formed after having observed a bunch of chickens. To Aristotle, the idea of a chicken was learned from observation of the unique combination of various physical qualities, what a scientist would define as the characteristics which define a particular species.

But how do the various elements in a chicken form it into chicken and not into a horse or a cat or some monstrous combination of all three? Plato argued that the idea of the chicken is separate from the body. Aristotle argued that the idea of a chicken is contained in its very substance. The form of a chicken is present in every different chicken and in each of its different parts. 

Similarly, Aristotle argued that nothing exists in our minds that we have not experienced with our senses through what we have heard and seen and read. Aristotle argued that we possess no innate ideas, but he does assert that all humans possess reason: the ability to organize and classify our sense impressions into categories and classes. In that way abstract ideas like 'chicken', 'stone', 'plant', 'animal', and 'human' have come into existence.

In a way, Plato has got him. Aristotle admits that reason is innate, but he argues that our minds are a blank slate until we have sensed something.

But how does Aristotle solve the problem of why a chicken turns into a chicken and not a cat, an elodile or a crocophant? He argues that all substances contain within them the potentiality to realize a particular form. He describes all change in nature as this tendency of particular substances to realize their forms, a movement from potential to actual. He conceived of a blueprint for life very similar to the DNA within a chromosome. (Which philosopher influenced him?) The plan for life which exists in each of our cells is Aristotle’s conception of the soul.

A chicken's egg cannot become a goose because that potentiality is not within its form. In the same way, it is in a stone's form (or programming) to fall to the ground if you drop it. 

Material Cause: the moisture of a cloud has the potential to condense 

Efficient Cause: the air cools

Formal Cause: It is in the nature of water to fall to earth

Final Cause: Rain possesses a "life purpose": to feed plants and animals

Yes, Aristotle believed that it rains and then oranges and grapes grow so that people can eat them. It is this final cause which demonstrates a purpose in nature: the existence of God. Today, we look at things from the other way round. Food and Water are necessary preconditions of life for plants and animals.

In his classifications of all material phenomena, Aristotle first distinguishes between living and non-living creatures. Only living things have a potentiality for change in their forms. 

In his classifications of living things, Aristotle use Plato's hierarchy to create his own understanding of life's categories: first between plants and creatures, and then between animals and humans.

At the bottom of the hierarchy are plants which possess the potential to absorb nourishment, to grow and to reproduce. (appetite)

Animals possess these potentialities, but they also can observe the world and move about. (volition)

Only humans also possess the ability to order their observations into different categories. Man grows and absorbs nourishment like plants (appetite), he has feelings and the ability to move like a creature (volition), but he also has a specific characteristic peculiar to humans: the ability to think rationally. (reason)

Aristotle conceived of the soul as the form of programming of our bodies. Man's soul has a plant-like aspect, an animal aspect, and a rational aspect. 

When he considered the question of how humans should live, Aristotle argued that man should pursue happiness. He conceived of three forms of happiness, which correspond to the three aspects of our soul: pleasure, freedom, and reason. For man to be happy, all three criteria need to be present and harmoniously balanced. We should not merely develop our body's strength or our rational skills, but we should find the Golden Mean between the two. 

Aristotle applied his conception of the Golden Mean to all of our characteristics. He found courage in the mean between cowardice and foolhardiness, liberality in the mean between miserliness and extravagance, health in the mean between starvation and gluttony.

Aristotle believed that man in its basic form is not a solitary creature but a political animal. We need family, village and state to best satisfy our potentialities. A family and a village can provide food, shelter, marriage and child rearing, but only the state can secure these needs.

Aristotle believed that various forms of government could achieve these goals, but the best finds the Golden Mean between tyranny and mob rule (anarchy). He referred to democracy as 'polity', but he also saw dangers in democracy which could lead to the tyranny of a majority.