Task: Question Answering
Input (English): Passage: Macedonia ( (listen); Greek: Μακεδονία, Makedonía), also called Macedon (), was an ancient kingdom on the periphery of Archaic and Classical Greece, and later the dominant state of Hellenistic Greece. The kingdom was founded and initially ruled by the royal Argead dynasty, which was followed by the Antipatrid and Antigonid dynasties. Home to the ancient Macedonians, the earliest kingdom was centered on the northeastern part of the Greek peninsula, and bordered by Epirus to the west, Paeonia to the north, Thrace to the east and Thessaly to the south.
Before the 4th century BC, Macedonia was a small kingdom outside of the area dominated by the great city-states of Athens, Sparta and Thebes, and briefly subordinate to Achaemenid Persia. During the reign of the Argead king Philip II (359–336 BC), Macedonia subdued mainland Greece and Thrace through conquest and diplomacy. With a reformed army containing phalanxes wielding the sarissa pike, Philip II defeated the old powers of Athens and Thebes in the Battle of Chaeronea in 338 BC. Philip II's son Alexander the Great, leading a federation of Greek states, accomplished his father's objective of commanding the whole of Greece when he destroyed Thebes after the city revolted. During Alexander's subsequent campaign of conquest, he overthrew the Achaemenid Empire and conquered territory that stretched as far as the Indus River. For a brief period, his empire was the most powerful in the world – the definitive Hellenistic state, inaugurating the transition to a new period of Ancient Greek civilization. Greek arts and literature flourished in the new conquered lands and advances in philosophy, engineering, and science spread throughout much of the ancient world. Of particular importance were the contributions of Aristotle, tutor to Alexander, whose writings became a keystone of Western philosophy.
After Alexander's death in 323 BC, the ensuing wars of the Diadochi, and the partitioning of Alexander's short-lived empire, Macedonia remained a Greek cultural and political center in the Mediterranean region along with Ptolemaic Egypt, the Seleucid Empire, and the Kingdom of Pergamon. Important cities such as Pella, Pydna, and Amphipolis were involved in power struggles for control of the territory. New cities were founded, such as Thessalonica by the usurper Cassander (named after his wife Thessalonike of Macedon). Macedonia's decline began with the Macedonian Wars and the rise of Rome as the leading Mediterranean power. At the end of the Third Macedonian War in 168 BC, the Macedonian monarchy was abolished and replaced by Roman client states. A short-lived revival of the monarchy during the Fourth Macedonian War in 150–148 BC ended with the establishment of the Roman province of Macedonia. 
Question: What is the name of the person whose writings became a keystone of Western philosophy?
Output (English): Aristotle.