A Protagonist of the Scientific Revolution:

Galileo Galilei was one of the protagonists of the scientific revolution, best known for his astronomical discoveries by means of a telescope (including sunspots, Jupiter’s satellites, and the phases of Venus), for his defense of heliocentrism, and for his study of the natural laws regarding falling bodies. Galileo, however, gave key contributions also to the development of the modern scientific methodology; for this reason, he holds a special place in the philosophy of science and epistemology.
Life:

Galileo was born in or nearby Pisa from Vincenzo Galilei and Giulia Ammannati. His father was a well known lutenist and music theorist, and the young Galileo was well versed in the art as well (Galileo’s youngest sibling, Michelangelo, became indeed an accomplished lutenist). During his youth, Galileo moved through different locations in Tuscany, among which the monastery of Vallombrosa, run by Camaldolese monks.

The story goes that until 1581 Galileo had not studied mathematics. After considering priesthood and enrolling into a program for physician at the University of Pisa, Galileo suddenly realized his mathematical vocation upon attending a geometry lecture. Immediately, he started discovering ingenious facts and creating crafts such as thermoscope. As early as 1589, Galileo held a chair in mathematics at the University of Pisa, which position he kept until 1592, when he moved to Padua. There, he remained until 1610.

The two most famous episodes of Galileo’s life concern his disputes with Roman Catholic authorities. The first took place in 1619, when Galileo entered into a direct controversy with father Orazio Grassi, of the Society of Jesus. The matter at issue concerned the nature of comets. The second dispute concerned heliocentrism and culminated in Galileo’s condemnation for heresy in 1633.
Sensed Experiences and Necessary Demonstrations:

Galileo’s success is due not only to the subtlety of his arguments, but also to the lucidity of his scientific methodology, well reflected also in the writing style. Philosophically speaking, the methodology is that part of Galileo’s work that is most relevant. He clearly demarcated between that information that is gathered through the senses (which he labeled "sensate esperienze," sensed experiences) and that which comes from deductive reasoning (labeled "necessary demonstrations").

The distinction is of great importance because it sets apart that which is understood by the intellect alone, independently of any specific experience, and that which depends on specific experiences in order to be known. Galileo was a strong defender of a form of Platonism, according to which the world is created according to certain mathematical laws, which humans can discover by properly using their rational capacities.
Subjective and Objective Properties:

Another key distinction drawn by Galileo sets apart those properties of objects that are due to the subjective characteristics of an agent and those that instead solely rest on the object’s own existence. Thus, that a certain feather is green and ticklish does depend on the specific subject (not everyone is ticklish, after all, and not experiences colors alike); on the other hand, the shape and weight of the feather will be true of it regardless of the subjective conditions of the agent who is set to explore them.

The reaffirmation of the distinction between subjective and objective properties will play a crucial role in the development of early modern philosophy: in cognate forms, it can be found for instance in authors as different as Descartes and Locke.

What was Galileo's significance to the scientific revolution?
Galileo was the first man to use a telescope to observe the stars and planets, and with the information he gained from this (craters on the moon, moons around Jupiter, sunspots), he was able to challenge the outdated view of the universe--which seemed to think that "the heavens" were perfect. Beyond astronomy, Galileo challenged Aristotle's concepts of motion, and discovered the principle of inertia, and his thoughts were la Aristotle

When we talk about Philosophy, the first name that comes into our mind is that of Aristotle (384 BC- 322 BC) who followed a comprehensive system of ideas about human nature and the nature of the reality we live in.

Early Life and Contributions:

One of the prominent names of history, this famous personality was a Greek philosopher, was born in Stagira in North Greece, the son of Nichomachus, the court physician to the Macedonian royal family. He was trained first in medicine, and then in 367BC was sent to Athens to study philosophy with Plato. He stayed at Plato’s Academy until about 347. He has also been under the supervision of Alexander the Great.

Aristotle is one of the most important founding figures in his time as his writings constitute a first at creating a broad system of Western philosophy, encompassing morality and aesthetics, logic and science, politics and metaphysics. Besides this his piece of work also includes other subjects, including physics, poetry, theater, music, rhetoric, government and ethics.

Though a bright pupil, Aristotle opposed some of Plato’s teachings, and when Plato died, Aristotle was not appointed head of the Academy. After leaving Athens, Aristotle spent some time traveling, and possibly studying biology, in Asia Minor and its islands. He returned to Macedonia in 338 to tutor Alexander the Great, after Alexander conquered Athens, Aristotle returned to Athens and set up a school of his own, known as the Lyceum. After Alexander’s death, Athens revolted against Macedonian rule, and Aristotle’s political situation became unstable. Therefore to keep away from being put to death, he fled to the island of Euboea, where he died soon after.

Legacy:
Now talking about Aristotle’s work and achievements, he was very versatile and his views on the physical sciences profoundly shaped medieval scholarship, and their influence extended well into the Renaissance, although they were ultimately replaced by Newtonian physics. In the biological sciences, some of his observations were confirmed to be accurate only for a few times. His works contain the earliest known formal study of logic, which was incorporated in the late nineteenth century into modern formal logic. A complete account of Aristotle’s contributions to science and philosophy is beyond the scope of this exhibit, but a brief summary can be made, whereas Aristotle’s teacher Plato had located ultimate reality in Ideas or eternal forms, knowable only through reflection and reason but on the other hand Aristotle saw final authenticity in physical matter, predictable through experience.

Matter has the potential to assume whatever form a sculptor gives it, and a seed or embryo has the potential to grow into a living plant or animal form. In living creatures, the form was known with the soul, plants had the lowest kinds of souls, animals had higher souls which could feel, and humans alone had rational, reasoning souls. In turn, animals could be classified by their way of life, their actions, or, most importantly, by their parts.

Though Aristotle’s work in zoology was not without faults, it was the grandest biological synthesis of the time, and remained the vital authority for many centuries after his death. His observations on the anatomy of octopus, cuttlefish, crustaceans, and many other marine invertebrates are extremely correct, with amazing results. He described the embryological development of a chick, and distinguished whales and dolphins from fish, plus he also noticed that some sharks give birth to live young. Aristotle’s books also discuss his detailed observations that he has been doing throughout his life.

We all have come across the classification of animals into different types and the readers will be amazed to know that Aristotle’s classification of animals grouped together is used in a much broader sense than present-day biologists use. He divided the animals into two types, those with blood, and those without blood (or at least without red blood). These distinctions correspond closely to our distinction between vertebrates and invertebrates. The blooded animals, corresponding to the vertebrates, whereas the bloodless animals were classified as cephalopods (such as the octopus), crustaceans, insects, shelled animals and zoophytes also known as plant-animals.

Aristotle’s thoughts on earth sciences can be found in his thesis Meteorology, the word today means the study of weather, but Aristotle used the word in a much broader sense, covering, as he put it, “all the affections we may call common to air and water, and the kinds and parts of the earth and the affections of its parts.” In it he discussed the nature of the earth and the oceans and explained the entire hydrologic cycle. The sun moving as it does sets up processes of change, and by its agency the finest and sweetest water is every day carried up and is dissolved into vapor and rises to the upper region, where it is condensed again by the cold and so returns to the earth.

He has also discussed winds, earthquakes, thunder, lightning, rainbows, meteors, comets, and the Milky Way. Aristotle was of the view that the whole vital process of the earth takes place so gradually and in periods of time which are so immense compared with the length of our life that these changes are not observed, and before their course can be recorded from beginning to end whole nations die and are ruined.

In metaphysics, Aristotelianism had a deep influence on philosophical and theological thinking in the Islamic and Jewish traditions in the Middle Ages, and it continues to influence Christian theology and the scholastic tradition of the Catholic Church. His followers called him Ille Philosophus (The Philosopher), or “the master of them that know,” and many accepted every word of his writings, or at least every word that did not contradict the Bible as eternal truth. All aspects of Aristotle’s philosophy continue to be the object of active academic study today.

Despite the far-reaching appeal that Aristotle’s works have traditionally enjoyed, today modern scholarship questions a considerable portion of the Aristotelian quantity as genuinely Aristotle’s own. Aristotle is said to have written 150 philosophical treatises. The 30 that survive touch on a huge range of philosophical problems, from biology and physics to morals to aesthetics to politics. Though Aristotle wrote many elegant treatises and dialogues, it is thought that the majority of his writings are now lost and only about one-third of the original works have endure but whatever has lasted is still a source of inspiration for the learners and will continue to be.ter incorporated into Isaac Newton's universal law of gravitation

Aristotelian physics

Aristotelian Physics, the natural sciences, are described in the works of the Greek philosopher Aristotle (384 BC – 322 BC). In the Physics, Aristotle established general principles of change that govern all natural bodies; both living and inanimate, celestial and terrestrial—including all motion, change in respect to place, change in respect to size or number, qualitative change of any kind, and coming to be and passing away. As Martin Heidegger, one of the foremost philosophers of the twentieth century, once wrote,
Aristotelian "physics" is different from what we mean today by this word, not only to the extent that it belongs to antiquity whereas the modern physical sciences belong to modernity, rather above all it is different by virtue of the fact that Aristotle's "physics" is philosophy, whereas modern physics is a positive science that presupposes a philosophy.... This book determines the warp and woof of the whole of Western thinking, even at that place where it, as modern thinking, appears to think at odds with ancient thinking. But opposition is invariably comprised of a decisive, and often even perilous, dependence. Without Aristotle's Physics there would have been no Galileo.[1]
To Aristotle, physics is a broad term that includes all nature sciences, such as philosophy of mind, body, sensory experience, memory and biology, and constitutes the foundational thinking underlying many of his works

Some concepts involved in Aristotle's physics are:
Teleology: Aristotle observes that natural things tend toward definite goals or ends insofar as they are natural. Regularities manifest a rudimentary kind of teleology.
Natural motion: Terrestrial objects tend toward a different part of the universe according to their composition of the four elements. For example, earth, the heaviest element, tends toward the center of the universe—hence the reason for the Earth being at the center. At the opposite extreme the lightest element, fire, tends upward, away from the center. The relative proportion of the four elements composing an object determines its motion. The elements are not proper substances in Aristotelian theory or the modern sense of the word. Refining an arbitrarily pure sample of an element isn't possible; They were abstractions; one might consider an arbitrarily pure sample of a terrestrial substance having a large ratio of one element relative to the others.
Terrestrial motion: Terrestrial objects move downward or upward toward their natural place. Motion from side to side results from the turbulent collision and sliding of the objects as well as transformations between the elements, (generation and corruption).
Rectilinear motion: Ideal terrestrial motion would proceed straight up or straight down at constant speed. Celestial motion is always ideal, it is circular and its speed is constant.
Speed, weight and resistance: The ideal speed of a terrestrial object is directly proportional to its weight. In nature, however, the matter obstructing an object's path is a limiting factor that's inversely proportional to the viscosity of the medium.
Vacuum isn't possible: Vacuum doesn't occur, but hypothetically, terrestrial motion in a vacuum would be indefinitely fast.
Continuum: Aristotle argues against the indivisibles of Democritus (which differ considerably from the historical and the modern use of the term atom).
Aether: The "greater and lesser lights of heaven", (the sun, moon, planets and stars), are embedded in perfectly concentric crystal spheres that rotate eternally at fixed rates. Because the spheres never change and (meteorites notwithstanding) don't fall down or rise up from the ground, they cannot be composed of the four terrestrial elements. Much as Homer's æthere (αἰθήρ), the "pure air" of Mount Olympus was the divine counterpart of the air (άήρ, aer) breathed by mortals, the celestial spheres are composed of a special element, eternal and unchanging, with circular natural motion.
Terrestrial change:

The four terrestrial elements
Unlike the eternal and unchanging celestial aether, each of the four terrestrial elements are capable of changing into either of the two elements they share a property with: e.g. the cold and wet (water) can transform into the hot and wet (air) or the cold and dry (earth) and any apparent change into the hot and dry (fire) is actually a two step process. These properties are predicated of an actual substance relative to the work it's able to do; that of heating or chilling and of desiccating or moistening. The four elements exist only with regard to this capacity and relative to some potential work. The celestial element is eternal and unchanging, so only the four terrestrial elements account for coming to be and passing away; also called "generation and corruption" after the Latin title of Aristotle's De Generatione et Corruptione (Περὶ γενέσεως καὶ φθορᾶ).
Celestial motion: The crystal spheres carrying the sun, moon and stars move eternally with unchanging circular motion. They're composed of solid aether and no gaps exist between the spheres. Spheres are embedded within spheres to account for the wandering stars, (i.e. the modern planets, which appear to move erratically in comparison to the sun, moon and stars). Later, the belief that all spheres are concentric was forsaken in favor of Ptolemy's deferent and epicycle. Aristotle submits to the calculations of astronomers regarding the total number of spheres and various accounts give a number in the neighborhood of 50 spheres. An unmoved mover is assumed for each sphere, including a prime mover for the sphere of fixed stars. The unmoved movers do not push the spheres (nor could they, they're insubstantial and dimensionless); rather, they're the final cause of the motion, meaning they explain it in a way that's similar to the explanation "the soul is moved by beauty". They simply "think about thinking", eternally without change, which is the idea of "being qua being" in Aristotle reformulation of Plato's theory.
While consistent with common human experience, Aristotle's principles were not based on controlled, quantitative experiments, so, while they account for many broad features of nature, they do not describe our universe in the precise, quantitative way we have more recently come to expect from science. Contemporaries of Aristotle like Aristarchus rejected these principles in favor of heliocentrism, but their ideas were not widely accepted. Aristotle's principles were difficult to disprove merely through casual everyday observation, but later development of the scientific method challenged his views with experiments, careful measurement, and more advanced technology such as the telescope and vacuum pump.
Elements
According to Aristotle, the elements which compose the terrestrial spheres are different from the one that composes the celestial spheres.[2] He believed that four elements make up everything under the moon (the terrestrial): earth, air, fire and water.[a][3] He also held that the heavens are made of a special, fifth element called "aether",[3] which is weightless and "incorruptible" (which is to say, it doesn't change).[3] Aether is also known by the name "quintessence"—literally, "fifth substance".[4]

Page from an 1837 edition of Physica by the ancient Greek philosopher Aristotle—a book about a variety of subjects including the philosophy of nature and some topics within physics
He considered heavy substances such as iron and other metals to consist primarily of the element earth, with a smaller amount of the other three terrestrial elements. Other, lighter objects, he believed, have less earth, relative to the other three elements in their composition.[4]

Motion
Aristotle held that each of the four terrestrial (or worldly) elements move toward their natural place, and that this natural motion would proceed unless hindered. For instance, because smoke is mainly air, it rises toward the sky but not as high as fire.

A vacuum, or void, is a place free of everything, and Aristotle argued against the possibility. Aristotle believed that the speed of an object's motion is proportional to the force being applied (or the object's weight in the case of natural motion) and inversely proportional to the viscosity of the medium; the more tenuous a medium is, the faster the motion. He reasoned that objects moving in a void, could move indefinitely fast and thus, the objects surrounding a void would immediately fill it before it could actually form.[5] In astronomy, voids, such as the Local Void adjacent to our galaxy, have the opposite effect; off-center bodies are ejected from the void due to the gravity of the material outside, which being the farthest away in a direction towards the center, is also at its weakest.[6]
Natural place
The Aristotelian explanation of gravity is that all bodies move toward their natural place. For the element earth, that place is the center of the (geocentric) universe, next comes the natural place of water (in a concentric shell around that of earth). The natural place of air is likewise a concentric shell surrounding the place of water. Sea level is between those two. Finally, the natural place of fire is higher than that of air but below the innermost celestial sphere, (the one carrying the Moon). Even at locations well above sea level, such as a mountain top, an object made mostly of the former two elements tends to fall and objects made mostly of the latter two tend to rise.
Place (topos)
In Book Delta of his Physics (IV.5) Aristotle defines topos (place) as the inner two-dimensional surface boundary of the containing body that is in touch with the outer two-dimensional surface of the contained body. This definition was the most dominant until the beginnings of the 17th century, even though it was called into doubt and debated by philosophers since antiquity, as for instance discussed by Simplicius in his Corollaries on place. The most significant early serious critique to this conception of place was demonstrated geometrically by the 11th century Arab polymath al-Hasan Ibn al-Haytham (Alhazen) in his Discourse on place.[7]

Some concepts involved in Aristotle's physics are:
Teleology: Aristotle observes that natural things tend toward definite goals or ends insofar as they are natural. Regularities manifest a rudimentary kind of teleology.
Natural motion: Terrestrial objects tend toward a different part of the universe according to their composition of the four elements. For example, earth, the heaviest element, tends toward the center of the universe—hence the reason for the Earth being at the center. At the opposite extreme the lightest element, fire, tends upward, away from the center. The relative proportion of the four elements composing an object determines its motion. The elements are not proper substances in Aristotelian theory or the modern sense of the word. Refining an arbitrarily pure sample of an element isn't possible; They were abstractions; one might consider an arbitrarily pure sample of a terrestrial substance having a large ratio of one element relative to the others.
Terrestrial motion: Terrestrial objects move downward or upward toward their natural place. Motion from side to side results from the turbulent collision and sliding of the objects as well as transformations between the elements, (generation and corruption).
Rectilinear motion: Ideal terrestrial motion would proceed straight up or straight down at constant speed. Celestial motion is always ideal, it is circular and its speed is constant.
Speed, weight and resistance: The ideal speed of a terrestrial object is directly proportional to its weight. In nature, however, the matter obstructing an object's path is a limiting factor that's inversely proportional to the viscosity of the medium.
Vacuum isn't possible: Vacuum doesn't occur, but hypothetically, terrestrial motion in a vacuum would be indefinitely fast.
Continuum: Aristotle argues against the indivisibles of Democritus (which differ considerably from the historical and the modern use of the term atom).
Aether: The "greater and lesser lights of heaven", (the sun, moon, planets and stars), are embedded in perfectly concentric crystal spheres that rotate eternally at fixed rates. Because the spheres never change and (meteorites notwithstanding) don't fall down or rise up from the ground, they cannot be composed of the four terrestrial elements. Much as Homer's æthere (αἰθήρ), the "pure air" of Mount Olympus was the divine counterpart of the air (άήρ, aer) breathed by mortals, the celestial spheres are composed of a special element, eternal and unchanging, with circular natural motion.
Terrestrial change:

The four terrestrial elements
Unlike the eternal and unchanging celestial aether, each of the four terrestrial elements are capable of changing into either of the two elements they share a property with: e.g. the cold and wet (water) can transform into the hot and wet (air) or the cold and dry (earth) and any apparent change into the hot and dry (fire) is actually a two step process. These properties are predicated of an actual substance relative to the work it's able to do; that of heating or chilling and of desiccating or moistening. The four elements exist only with regard to this capacity and relative to some potential work. The celestial element is eternal and unchanging, so only the four terrestrial elements account for coming to be and passing away; also called "generation and corruption" after the Latin title of Aristotle's De Generatione et Corruptione (Περὶ γενέσεως καὶ φθορᾶ).
Celestial motion: The crystal spheres carrying the sun, moon and stars move eternally with unchanging circular motion. They're composed of solid aether and no gaps exist between the spheres. Spheres are embedded within spheres to account for the wandering stars, (i.e. the modern planets, which appear to move erratically in comparison to the sun, moon and stars). Later, the belief that all spheres are concentric was forsaken in favor of Ptolemy's deferent and epicycle. Aristotle submits to the calculations of astronomers regarding the total number of spheres and various accounts give a number in the neighborhood of 50 spheres. An unmoved mover is assumed for each sphere, including a prime mover for the sphere of fixed stars. The unmoved movers do not push the spheres (nor could they, they're insubstantial and dimensionless); rather, they're the final cause of the motion, meaning they explain it in a way that's similar to the explanation "the soul is moved by beauty". They simply "think about thinking", eternally without change, which is the idea of "being qua being" in Aristotle reformulation of Plato's theory.
While consistent with common human experience, Aristotle's principles were not based on controlled, quantitative experiments, so, while they account for many broad features of nature, they do not describe our universe in the precise, quantitative way we have more recently come to expect from science. Contemporaries of Aristotle like Aristarchus rejected these principles in favor of heliocentrism, but their ideas were not widely accepted. Aristotle's principles were difficult to disprove merely through casual everyday observation, but later development of the scientific method challenged his views with experiments, careful measurement, and more advanced technology such as the telescope and vacuum pump.
Elements [edit]
According to Aristotle, the elements which compose the terrestrial spheres are different from the one that composes the celestial spheres.[2] He believed that four elements make up everything under the moon (the terrestrial): earth, air, fire and water.[a][3] He also held that the heavens are made of a special, fifth element called "aether",[3] which is weightless and "incorruptible" (which is to say, it doesn't change).[3] Aether is also known by the name "quintessence"—literally, "fifth substance".[4]

Page from an 1837 edition of Physica by the ancient Greek philosopher Aristotle—a book about a variety of subjects including the philosophy of nature and some topics within physics
He considered heavy substances such as iron and other metals to consist primarily of the element earth, with a smaller amount of the other three terrestrial elements. Other, lighter objects, he believed, have less earth, relative to the other three elements in their composition.[4]

Motion
Aristotle held that each of the four terrestrial (or worldly) elements move toward their natural place, and that this natural motion would proceed unless hindered. For instance, because smoke is mainly air, it rises toward the sky but not as high as fire.

A vacuum, or void, is a place free of everything, and Aristotle argued against the possibility. Aristotle believed that the speed of an object's motion is proportional to the force being applied (or the object's weight in the case of natural motion) and inversely proportional to the viscosity of the medium; the more tenuous a medium is, the faster the motion. He reasoned that objects moving in a void, could move indefinitely fast and thus, the objects surrounding a void would immediately fill it before it could actually form.[5] In astronomy, voids, such as the Local Void adjacent to our galaxy, have the opposite effect; off-center bodies are ejected from the void due to the gravity of the material outside, which being the farthest away in a direction towards the center, is also at its weakest.[6]
Natural place
The Aristotelian explanation of gravity is that all bodies move toward their natural place. For the element earth, that place is the center of the (geocentric) universe, next comes the natural place of water (in a concentric shell around that of earth). The natural place of air is likewise a concentric shell surrounding the place of water. Sea level is between those two. Finally, the natural place of fire is higher than that of air but below the innermost celestial sphere, (the one carrying the Moon). Even at locations well above sea level, such as a mountain top, an object made mostly of the former two elements tends to fall and objects made mostly of the latter two tend to rise.
Place (topos) [edit]
In Book Delta of his Physics (IV.5) Aristotle defines topos (place) as the inner two-dimensional surface boundary of the containing body that is in touch with the outer two-dimensional surface of the contained body. This definition was the most dominant until the beginnings of the 17th century, even though it was called into doubt and debated by philosophers since antiquity, as for instance discussed by Simplicius in his Corollaries on place. The most significant early serious critique to this conception of place was demonstrated geometrically by the 11th century Arab polymath al-Hasan Ibn al-Haytham (Alhazen) in his Discourse on place.[7]

Theory of impetus
The Aristotelian theory of motion came under criticism and/or modification during the Middle Ages. The first such modification came from John Philoponus in the 6th century. He partly accepted Aristotle's theory that "continuation of motion depends on continued action of a force," but modified it to include his idea that the hurled body acquires a motive power or inclination for forced movement from the agent producing the initial motion and that this power secures the continuation of such motion. However, he argued that this impressed virtue was temporary; that it was a self-expending inclination, and thus the violent motion produced comes to an end, changing back into natural motion. In the 11th century, the Persian polymath Avicenna, in The Book of Healing (1027) was influenced by Philoponus' theory in its rough outline, but took it much further to present the first alternative to the Aristotelian theory. In the Avicennan theory of motion, the violent inclination he conceived was non-self-consuming, a permanent force whose effect was dissipated only as a result of external agents such as air resistance, making him "the first to conceive such a permanent type of impressed virtue for non-natural motion." Such a self-motion (mayl) is "almost the opposite of the Aristotelian conception of violent motion of the projectile type, and it is rather reminiscent of the principle of inertia, i.e., Newton's first law of motion."[8]
The eldest Banū Mūsā brother, Ja'far Muhammad ibn Mūsā ibn Shākir (800-873), wrote the Astral Motion and The Force of Attraction. The Persian physicist, Ibn al-Haytham (965-1039), discussed the theory of attraction between bodies. It seems that he was aware of the magnitude of acceleration due to gravity and he discovered that the heavenly bodies "were accountable to the laws of physics".[9] The Persian polymath Abū Rayhān al-Bīrūnī (973-1048) was the first to realize that acceleration is connected with non-uniform motion, part of Newton's second law of motion.[10] During his debate with Avicenna, al-Biruni also criticized the Aristotelian theory of gravity for denying the existence of levity or gravity in the celestial spheres and for its notion of circular motion being an innate property of the heavenly bodies.[11]
In 1121, al-Khazini, in The Book of the Balance of Wisdom, proposed that the gravity and gravitational potential energy of a body varies depending on its distance from the centre of the Earth.[12][not in citation given] Hibat Allah Abu'l-Barakat al-Baghdaadi (1080–1165) wrote a critique of Aristotelian physics entitled al-Mu'tabar, where he negated Aristotle's idea that a constant force produces uniform motion, as he realized that a force applied continuously produces acceleration, a fundamental law of classical mechanics and an early foreshadowing of Newton's second law of motion.[13] Like Newton, he described acceleration as the rate of change of speed.[14]
In the 14th century, Jean Buridan developed the theory of impetus as an alternative to the Aristotelian theory of motion. The theory of impetus was a precursor to the concepts of inertia and momentum in classical mechanics.[15] Buridan and Albert of Saxony also refer to Abu'l-Barakat in explaining that the acceleration of a falling body is a result of its increasing impetus.[16] In the 16th century, Al-Birjandi discussed the possibility of the Earth's rotation. In his analysis of what might occur if the Earth were rotating, he developed a hypothesis similar to Galileo Galilei's notion of "circular inertia",[17] which he described in the following observational test:
"The small or large rock will fall to the Earth along the path of a line that is perpendicular to the plane (sath) of the horizon; this is witnessed by experience (tajriba). And this perpendicular is away from the tangent point of the Earth’s sphere and the plane of the perceived (hissi) horizon. This point moves with the motion of the Earth and thus there will be no difference in place of fall of the two rocks."[18]
Life and death of Aristotelian physics [edit]

The famous philosopher Aristotle, depicted in a painting by Rembrandt Harmensz. van Rijn
The reign of Aristotelian physics lasted for almost two millennia, and provides the earliest known speculative theories of physics. After the work of Galileo, Descartes, and many others, it became generally accepted that Aristotelian physics was not correct or viable.[4] Despite this, the scholastic science survived well into the seventeenth century, and perhaps even later, until universities amended their curricula.
In Europe, Aristotle's theory was first convincingly discredited by the work of Galileo Galilei. Using a telescope, Galileo observed that the moon was not entirely smooth, but had craters and mountains, contradicting the Aristotelian idea of an incorruptible perfectly smooth moon. Galileo also criticized this notion theoretically – a perfectly smooth moon would reflect light unevenly like a shiny billiard ball, so that the edges of the moon's disk would have a different brightness than the point where a tangent plane reflects sunlight directly to the eye. A rough moon reflects in all directions equally, leading to a disk of approximately equal brightness which is what is observed.[19] Galileo also observed that Jupiter has moons, objects which revolve around a body other than the Earth. He noted the phases of Venus, convincingly demonstrating that Venus, and by implication Mercury, travels around the sun, not the Earth.
According to legend, Galileo dropped balls of various densities from the Tower of Pisa and found that lighter and heavier ones fell at almost the same speed. In fact, he did quantitative experiments with balls rolling down an inclined plane, a form of falling that is slow enough to be measured without advanced instruments.
A heavier body falls faster than a lighter one of the same shape in a dense medium like water, and this led Aristotle to speculate that the rate of falling is proportional to the weight and inversely proportional to the density of the medium. From his experience with objects falling in water, he concluded that water is approximately ten times denser than air. By weighing a volume of compressed air, Galileo showed that this overestimates the density of air by a factor of forty.[20] From his experiments with inclined planes, he concluded that all bodies fall at the same rate neglecting friction.
Galileo also advanced a theoretical argument to support his conclusion. He asked if two bodies of different weights and different rates of fall are tied by a string, does the combined system fall faster because it is now more massive, or does the lighter body in its slower fall hold back the heavier body? The only convincing answer is neither: all the systems fall at the same rate.[19]
Followers of Aristotle were aware that the motion of falling bodies was not uniform, but picked up speed with time. Since time is an abstract quantity, the peripatetics postulated that the speed was proportional to the distance. Galileo established experimentally that the speed is proportional to the time, but he also gave a theoretical argument that the speed could not possibly be proportional to the distance. In modern terms, if the rate of fall is proportional to the distance, the differential equation for the distance y travelled after time t is

with the condition that . Galileo demonstrated that this system would stay at for all time. If a perturbation set the system into motion somehow, the object would pick up speed exponentially in time, not quadratically.[20]
Standing on the surface of the moon in 1971, David Scott famously repeated Galileo's experiment by dropping a feather and a hammer from each hand at the same time. In the absence of a substantial atmosphere, the two objects fell and hit the moon's surface at the same time.
With his law of universal gravitation Isaac Newton was the first to mathematically codify a correct theory of gravity. In this theory, any mass is attracted to any other mass by a force which decreases as the inverse square of their distance. In 1915, Newton's theory was replaced by Albert Einstein's general theory of relativity. See gravity for a much more detailed complete discussion.