Speaking of Aquinas, perhaps this is an opportune moment to take a look at Aristotle.
The diametric differences in approach between Aristotle and Plato, Aristotle's teacher at the Academy in Athens, are deeply embedded in much of what Western philosophers have bickered about for centuries since -- endless variations on the themes of Plato's belief in an objective reality existing apart from human sensation, and Aristotle's emphasis on intelligent observation of physical phenomena.
An orientation toward the physical may have been cooked into Aristotle's veins: his father Nicomachus was physician to the Macedonian king Amyntas II, and it is said that he began training Aristotle to follow in the profession from a young age. Nicomachus died when Aristotle was 10, but certainly there may have been some attempt on the part of Aristotle's guardian to carry out Nicomachus' wishes in sending the 17-year old to study in Plato's famous Academy. Although Plato was in Syracuse when Aristotle arrived, it did not take him long after his return to identify Aristotle as a young man of superior capabilities, and soon he began to call young Aristotle "the intellect of the school." Aristotle eventually became a teacher at the Academy, possibly instructing in rhetoric and dialectic -- the art of question and answer.
When Aristotle was 37, Plato died, leaving the Academy in the hands of his mathematically-oriented nephew Speusippus. Numbers not being Aristotle's cup of tea, he left the Academy. His old ties with powerful Macedonia (including with its ruler, his boyhood friend Philip) made him a marketable commodity, and he soon accepted the invitation of Hermias, king of Atarneus, to teach. Hermias, seen by some as a living embodiment of Plato's "philosopher-king," certainly saw the advantages of keeping the well-affiliated Macedonian around, and married his niece to Aristotle. Despite his loyalty to Hermias, Aristotle began a now-lost treatise at this time which suggested that it was a disadvantage for a king to be a philosopher, that instead a king who relied on the fresh perspectives of wise philosophers would be more effective than Plato's philosopher-king.
It is unclear whether the shortcomings of Hermias' own intellect had anything to do with it, but in 344 B.C. Hermias was captured and executed by invading Persians. Aristotle fled to the island of Lesbos, where in exile he returned to his childhood anatomy lessons, gathering specimens of marine life along the island's shore, inspecting them with his scalpel and recording copious notes about them. The idyll proved to have a profound influence on Aristotle's thought, gradually turning him away from Plato's concept of a higher plane of reality and back to the present fundamentals -- the scent of mollusks and sea air, the taste of salt water on his skin, the rush of the wind and the heat of the Aegean sun.
He left Lesbos for Macedonia after a couple of years, by legend to tutor Philip's son, the future Alexander the Great. Whether or not he was Alexander's tutor remains a matter of conjecture, but the close connections between the two are not: Alexander later saw to it that local specimens of flora and fauna from his conquests were sent back to Aristotle's library, and when Alexander became king in 336 B.C., he funded Aristotle's launch of the Lyceum in Athens, a rival school to the Academy.
Rather than engaging students in dialogues as Plato did, Aristotle preferred to lecture, wandering through the halls of the Lyceum with students gathered in tow. More importantly, however, Aristotle expanded the scope of education, defining new disciplines such as biology and psychology and lecturing on everything from politics and ethics to metaphysics, economics and rhetoric. It is the Aristotle of the Lyceum -- the squat, pinky-ring wearing, spindly-legged man in his late 40s and 50s, exceptionally well-organized and an avid collector of all manner of material things -- that we have incidentally come to know best.
The only texts by Aristotle which survive are his lecture notes from this period, which is a little like trying to get to know Dickens from a set of storyboard sketches, perhaps. Nonetheless, Aristotle's lecture notes reveal his concern with breaking things (or questions) down to their constituent parts for more manageable data analysis, with classification so that information could be easily accessed (his classifications of plants and animal life still form the basis of modern biological classification) and with the extraction of knowledge through the use of logic.
Principal among his works are Organon, on logic, illustrating the understanding of scientific questions through the implementation of systems of axioms; Physics, a review of astronomy, meteorology, plants and animals; First Philosophy, or Metaphysics, an analysis of the most general, abstract features of reality, the principles which define existence (including a description of Aristotle's God-like "Prime Mover," the first cause of all existence); his work on ethics, dedicated to his son Nico, the Nicomachean Ethics; Rhetoric; and Poetics.
His methodologies for understanding existence were derived from observation. His description of four causes for every real thing in nature (the material, based on a thing's component substances; the efficient, or the process which produces a thing; the formal, a thing's ultimate shape; and the final, that which gives it its purpose or meaning) and 10 categories of being (substance, quantity, quality, relation, place, time, position, state, action and affection) are quite far removed from Plato's extrasensory interests, revealing the mind of a diagnostic mechanic. If Plato might want to engage you in a discussion of your "stuff" with the aim of turning you toward unseen absolutes, Aristotle would not be able to keep his paws off your "stuff," taking it apart and putting it back together and showing you things you never guessed you'd ever know about your "stuff."
While the enveloping sense of authority throughout Aristotle's writings may be seen as somewhat self-important and overbearing (especially to his enemies), the old teacher is thought to have been generous and affectionate. When Alexander died in 323 B.C., Aristotle's enemies mobilized the anti-Macedonian sentiment in Athens and officially charged Aristotle with the crime of impiety. Recalling the fate of Socrates and his failed attempt to defend himself from similar false charges, Aristotle fled Athens for Chalcis, and died there of a stomach ailment after only a few months. His body was taken back to his hometown of Stagira and his ashes were buried with great honors.
Meanwhile, the Lyceum remained open until 529 A.D. when Justinian closed all "philosophy" schools. Aristotle's influence on the intellectual histories of Islam and the West has been pervasive: his work, particularly his Metaphysics and his rational model for the existence of a Prime Mover, was commented upon and engaged by such diverse writers as Ibn Sina, Ibn Rushd, Aquinas and Maimonides; and his conceptions of biology and astronomy defined most of what came afterwards, even as the empirical value of his conclusions have gradually eroded since the Renaissance.