‘My King, the Earth is a stationary sphere, surrounded by multiple layers of celestial bodies: closest to the Earth is the Moon, followed by the Sun and the five planets, and finally the countless fixed stars. These celestial bodies move along a transparent sphere that revolves with the Earth at its centre.’

The man who was speaking so passionately to Alexander the Great about the laws governing the motion of the heavens was Aristotle, the last man of his time to be a master of all disciplines and possess such vast wisdom.

Let us go back to 384 BC, to the city of Stagira in ancient Greece, where Aristotle was born. His father, Nicomachus, was the court physician to Philip II of Macedon; consequently, Aristotle grew up in an aristocratic household.

It should be noted here that this Kingdom of Macedonia later developed into the Empire of Alexander the Great with which we are familiar; however, at this time, the reins of power still lay in the hands of Alexander the Great’s father.

When Aristotle was 18 years old, he was sent to Plato’s Academy in Athens, where he studied and lived. He remained there for 20 years, until 347 BC, when his mentor Plato died and he chose to leave Athens; it can be said that the Academy witnessed his growth.

After leaving the Academy, he accepted an invitation from Hermias, a former fellow student and ruler of Mysia on the coast of Asia Minor, to travel to Assos in Asia Minor. There, Aristotle married Hermias’s niece, Pisias.

In 344 BC, Hermias was killed in an uprising, forcing Aristotle to leave Asia Minor and travel with his family to Mytilene. After living there for two years, Aristotle was summoned back to his homeland by Philip II to become the tutor to his son, Alexander.

According to the renowned ancient Greek biographer Plutarch, Aristotle imparted a wealth of knowledge—including ethics, politics and philosophy—to this future world leader; this knowledge had a significant influence on the thinking of Alexander the Great and inspired his pursuit of science and knowledge.

Although the two did not entirely share the same political views, they nevertheless respected one another, and Alexander the Great provided substantial human and financial resources to support Aristotle’s various research endeavours.

Following the death of Philip II, Alexander ascended the throne in 336 BC, whereupon Aristotle established his own academy on the outskirts of Athens, where he continued his teaching and research. Aristotle had a habit of ‘teaching whilst walking’; he would stroll through the corridors and gardens whilst lecturing.

It is for this reason that his philosophy came to be known as the ‘Peripatetic philosophy’ or the ‘philosophy of walking’, and his followers were known as the ‘Peripatetics’, or ‘walkers’.

Aristotle’s scholarship spanned virtually every discipline of his time, including anatomy, astronomy, economics, embryology, geography, geology, meteorology, physics, zoology, aesthetics, ethics, political science, metaphysics, psychology and theology, amongst others.

Furthermore, Aristotle also ventured into the fields of education, literature and poetry. His works encompassed virtually every aspect of ancient Greek knowledge, constituting, in effect, an encyclopaedia of the intellectual world of his time.

Following Alexander’s death, the people of Athens began to resist the rule of the Kingdom of Macedonia.

Owing to his relationship with Alexander as his teacher, Aristotle was accused of impiety and forced to flee to Chalcis for refuge. His Academy was entrusted to the care of Theophrastus. Aristotle explained the reason for his departure as follows: ‘I do not wish the Athenians to commit a second sin of destroying philosophy.’

This was, in fact, an allusion to the death of Socrates. However, one year later, in 322 BC, Aristotle died at the age of 62 from illnesses that had accumulated over many years.

In the field of natural philosophy, he proposed the theory of the five elements, positing that the world was composed of the following five fundamental elements:

Earth: possessing the qualities of cold and dryness, corresponding to the modern concept of solids.

Water: exhibiting the properties of cold and wetness, corresponding to the modern concept of a liquid.

Fire: exhibiting the properties of heat and dryness, equivalent to the modern concept of heat.

Air: possessing the properties of heat and wetness, corresponding to the modern concept of a gas.

Ether: the divine substance constituting the celestial spheres and celestial bodies (such as stars and planets).

In Aristotle’s view, the Earth is the centre of the universe, followed by water, air and then fire. These elements undergo a natural cyclical motion without the need for an external driving force. Consequently, the human body sinks in water; water evaporates into air and then falls as rain; and fire can burn in the air. The cyclical motion of these elements is eternal and unchanging.

Aristotle believed that the celestial bodies above the Moon were fundamentally different from the objects below the Earth.

Objects below the Moon are composed of the four elements, each of which possesses different properties in different regions. Closest to the Moon is the region of the element of fire, followed by the regions of the elements of air, water and earth. These elements move naturally according to their nature; for example, earth moves towards the centre of the universe, whilst fire moves away from it.

This explains why there are distinct regions between the Moon and the Earth.

Above the Moon, the celestial bodies are composed of another element known as the aether. The aether is an element that moves in a uniform circular motion that is eternal and unchanging; unlike other elements, it does not transform into other elements or exchange positions with them. This explains why the celestial bodies are able to move in a uniform circular motion that is eternal and unchanging.

To account for the constant changes in objects beneath the Earth, Aristotle posited the need for an eternal prime mover, a role he ascribed to the celestial bodies. Owing to the motion of the celestial bodies, the spheres in direct contact with them are constantly driven to rotate, thereby influencing the objects beneath the Earth.

Aristotle believed that to fully understand the complex motions of the celestial bodies, one must take into account the roles of thought and desire, as he regarded the celestial bodies as possessing intelligence and life.

Aristotle defined metaphysics as the knowledge of ‘immaterial existence’ or ‘existence at the highest level of abstraction’, and, together with theology, referred to it as ‘First Philosophy’.

He also put forward the theory of the four causes to explain causal relationships in the natural world:

Material cause: Things are composed of parts, components, foundations or raw materials, representing the material constitution of things.

Formal cause: This explains the definition, form, shape, essence or archetype of a thing, and is the fundamental principle of its constitution.

Efficient cause: This governs the change and origin of a thing, including the source of the force that alters its state.

Final cause: This explains the purpose of a thing’s existence or change, including purposeful actions and activities.

In his Metaphysics, he explored the concept of ‘substance’ in depth, arguing that the substance of a particular thing arises from the union of form and matter. He categorised change into three types: growth or shrinkage, motion, and a change in the overall quality of a thing, and introduced the concepts of ‘potentiality’ and ‘actuality’.

‘Potentiality’ denotes the capacity of a particular thing to undergo a change, assuming no external force prevents it. ‘Actuality’, on the other hand, is the result of potentiality being realised, and is also the fundamental principle underlying all changes in things.

Taking a house as an example, its ‘matter’ is potentiality, whilst the house’s ‘form’ is actuality. The architect who draws up the plans is the formal cause; reason is what transforms potentiality into an actual house; and the completed house itself is the final cause.

Aristotle pointed out that actuality precedes potentiality both in time and in substance. This inquiry is not limited to the material world but also encompasses abstract concepts and ideas. Aristotle regarded metaphysics as the study of the highest forms of being and the highest realm in the pursuit of wisdom and truth. These factors can influence one another, leading to changes in outcomes.

In the realm of philosophy, Aristotle occupies a position between idealism and materialism. He defined his philosophy as ‘causality’, claiming it to be ‘the science that investigates the true causes of the universe’. Although Aristotle harboured no doubt regarding the reality of the external world—a stance closer to materialism—he simultaneously held that the foundation and intrinsic essence of all things lie in form, whilst matter is merely their secondary manifestation.

Consequently, he maintained that form precedes matter, thereby embodying an idealist perspective.

By contrast, Plato defined his philosophy as the ‘science of ideas’, where ‘ideas’ refer to the fundamental principles underlying all phenomena. Both philosophers regarded philosophy as the study of the universe, but Aristotle achieved this by examining the essence of particular things, whilst Plato held that the universe had no direct connection with particular things, the latter being merely specimens or models of the universe.

Aristotle viewed philosophy as a process of ascending from the study of specific phenomena to the study of the essence of things, whilst Plato regarded it as a process of ascending from the study of universal Ideas to the study of the specific specimens into which these Ideas are transformed. Aristotle employed inductive and deductive methods, whereas Plato’s approach to inquiry was rooted in a priori principles.

In Aristotle’s view, ‘natural philosophy’ referred to the study of the natural world, including motion, light and the laws of physics. These fields later became the foundation of modern science, studied through the scientific method. In modern times, however, the term ‘philosophy’ is typically applied to the realm of metaphysics rather than to the study of the natural world.

In his Politics, written two thousand years ago, he predicted the advent of the Industrial Revolution, describing an automated world in which every machine could operate independently, carrying out tasks without the need for human intervention. This forward-looking perspective demonstrates his profound insight into the future development of society.

Aristotle’s ideas have had a profound influence on Western philosophy and science, and he is regarded as one of the key founders of both ancient and modern philosophy.

The medieval English poet Chaucer once described one of his students as follows:

On his bed lay twenty books with black or red covers, containing Aristotle and his philosophy.

Even in the modern era, Aristotle’s ideas continue to attract considerable attention and debate; his works are regarded as classics of Western culture and philosophy, exerting a lasting influence on subsequent philosophical, scientific and political theories.