Philoponus’ main significance for the history of science lies in his being, at the close of antiquity, the first thinker to undertake a comprehensive and massive attack on the principal tenets of Aristotle’s physics and cosmology, an attack unequaled in thoroughness until Galileo.He argued that the sun is fire and of terrestrial-like, corruptible matter. He devised a precursor to the notion of impetus which Buridan later developed, that which keeps moving bodies in motion even after the mover ceases being in contact with them; air does not keep projectiles in motion. He discovered that light rays travel the same both backwards and forwards. He invented functions of variables and their "courses" (what we'd call "first derivatives" in modern calculus). He discovered the law of inertia, that bodies in motion remain in motion unless something impedes their movement, literally a thousand years before Galileo, Newton, et al.!
He's certainly one of the "grands génies de l'Antiquité" ("great geniuses of Antiquity") and "principaux précurseurs de la Science moderne" ("principle precursers to modern Science"), as Pierre Duhem wrote.
Philoponus's arguments on the non-eternity of the world were interesting. From the Encyclopedia of Time: Science, Philosophy, Theology, & Culture (see also his Dict. of Sci. Bio. entry):
Philoponus and Simplicius
The beginning or the eternity of the world and infinity or finitude of time is a central topic in the philosophy of late antiquity, especially in the debate between Christians and pagans. This quarrel started for the first time in the Neoplatonic school of Alexandria in the 6th century CE between John Philoponus and Simplicius, who were the philosophically most talented pupils of Ammonius Hermeiou. Simplicius preserved the orthodox Neoplatonic doctrine (Ammonius and his master Proclus always held to the eternity of the world), whereas the Christian Philoponus opposed this view. It is astonishing that the grammarian Philoponus (he called himself John the Grammarian and edited most of Ammonius's lectures on Aristotle's writings) argued without Christian presuppositions and personal Philoponus and disparagement; Simplicius, however, usually a very modest and well-educated philosopher, very rudely called Philoponus's arguments “rubbish” and accused him of “bragging and contentiousness.” Obviously, they had never met personally (most probably, Simplicius had been working in Athens long before 529 CE, when the academy was closed by Justinian; Philoponus apparently never left Alexandria). Philoponus argued against the eternity of the world in his commentaries on Aristotle's Physics (probably written in 517 CE) and Meteorology, then in On the Eternity of the World, Against Proclus (De aeternitate mundi contra Proclum, written in 529 CE; this treatise refutes 18 arguments from a lost treatise written by Proclus about the eternity of the world). The writing Against Aristotle (Contra Aristotelem), which can be dated between approximately 530 CE and 534 CE, is preserved only in fragments. The first five books contained Philoponus's criticism of Aristotle's theory of the fifth element, the sixth book his criticism of Aristotle's theory of eternal movement, and at least two further books contained reflections about a Christian theory of divine creation. Simplicius's answer to Philoponus can be found mainly in his commentary on Aristotle's De caelo I and Physics VIII
Philoponus attacks the eternity of the world by demonstrating inner contradictions in Aristotle's theory of time and eternity and by refuting Aristotle through Aristotle himself. One argument goes as follows: The eternity of the world is incompatible with Aristotle's definition of movement, because movement is the act of what is movable in potency, that is, the movable in potency exists prior to the movement. This implies that the eternal movements (e.g., the heavens' circular movements) have some movable in potency prior to them (e.g., the heavens), if the movable in potency is always anterior to the movement. Philoponus concludes that the Aristotelian definition of movement is not universal. Simplicius defends the universality of Aristotle's definition of movement by making a difference between infinite and finite movement: In the case of finite movement, the movable is still there, if the movement has finished; in the case of infinite, eternal movement, only one state of movement is prior to another state. For instance, if the sun is in Aries, then it is the movable, which is potentially in Taurus.
Further, if any first movement is excluded, Philoponus argues that all present movements become unintelligible, because every movement presupposes an infinite number of previous movements; we could not avoid a regressus in infinitum. Moreover, all present movements are added to those of the past; that leads to the evidently absurd notion of an infinite constantly increasing. The same problem arises concerning the future: If time and movement infinitely continue in the future, there would be an infinite body with infinite power. But that is not possible, so the world could not exist indefinitely in the future. The core of this argument is Philoponus's attack on Aristotle's notion of infinity: Aristotle contends that infinity is merely potential and never actual. For if you divide a line or a duration, you can actually mark off only a finite number of divisions, either physically or mentally. There is only a potential infinity of divisions, inasmuch as infinity exists through a process of dividing one point (or one now) after another; it is the same with the infinity of numbers.
Philoponus attacks this notion of infinity by several arguments. First, the universe must have had a beginning, or it would by now have traversed an actual infinity of years. The second argument is this: If you suppose an actual infinite number of years up to this year, next year will be an infinity plus one year. So the infinity is increasing. Simplicius says Aristotle had already anticipated Philoponus's objections, for he had pointed out that the past years have finished, so you do not get an actual infinity of them existing. That implies that time and movement are not an actually infinite quantity, but their infinity means there is a possibility of transcending every given limitation. The most fundamental difference between Philoponus and Simplicius is this: Whereas, Simplicius's infinite time is a circular indefinite repetition of finite times, Philoponus's notion of time is linear. However, the rejection of Philoponus's argument appears difficult in the context of Aristotle's philosophy of nature if you want to preserve the singularity of the individual parts of time, for instance, days or hours.
Philoponus's arguments against the eternity of the world were repeated by Bonaventure in the 13th century, after the arguments had been elaborated by Islamic philosophers. Finally, the dispute between Philoponus and Simplicius has an equivalent in Kant's doctrine of the “antinomy of pure reason” in his Critique of Pure Reason, especially the “first conflict of transcendental ideas.” One branch of the antinomy is equivalent to Philoponus's argument (in Kant's words, “The world has a beginning in time”); this and the opposite argument (“the world has no beginning in time”) is equivalent to Simplicius. Probably, there is not any “ immediate effective historical connection” between the Alexandrian school quarrel and Kant's cosmo-logical antinomy, but it shows that in this quarrel, “Greek thinking comes to the limits of its own presuppositions.”