instruction:
In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
question:
What is needed to prove the theory?, Context: In the early 11th century, the Muslim physicist Ibn al-Haytham (Alhacen or Alhazen) discussed space perception and its epistemological implications in his Book of Optics (1021), he also rejected Aristotle's definition of topos (Physics IV) by way of geometric demonstrations and defined place as a mathematical spatial extension. His experimental proof of the intromission model of vision led to changes in the understanding of the visual perception of space, contrary to the previous emission theory of vision supported by Euclid and Ptolemy. In "tying the visual perception of space to prior bodily experience, Alhacen unequivocally rejected the intuitiveness of spatial perception and, therefore, the autonomy of vision. Without tangible notions of distance and size for correlation, sight can tell us next to nothing about such things."
answer:
vision


question:
Raiders can also be called?, Context: Perhaps the most famous raid by Oeselian pirates occurred in 1187, with the attack on the Swedish town of Sigtuna by Finnic raiders from Couronia and Oesel. Among the casualties of this raid was the Swedish archbishop Johannes. The city remained occupied for some time, contributing to its decline as a center of commerce in the 13th century and the rise of Uppsala, Visby, Kalmar and Stockholm. The Livonian Chronicle describes the Oeselians as using two kinds of ships, the piratica and the liburna. The former was a warship, the latter mainly a merchant ship. A piratica could carry approximately 30 men and had a high prow shaped like a dragon or a snakehead and a rectangular sail. Viking-age treasures from Estonia mostly contain silver coins and bars. Saaremaa has the richest finds of Viking treasures after Gotland in Sweden. This strongly suggests that Estonia was an important transit country during the Viking era.
answer:
pirates


question:
What initiated the liquidity crisis?, Context: It threatened the collapse of large financial institutions, which was prevented by the bailout of banks by national governments, but stock markets still dropped worldwide. In many areas, the housing market also suffered, resulting in evictions, foreclosures and prolonged unemployment. The crisis played a significant role in the failure of key businesses, declines in consumer wealth estimated in trillions of U.S. dollars, and a downturn in economic activity leading to the 2008–2012 global recession and contributing to the European sovereign-debt crisis. The active phase of the crisis, which manifested as a liquidity crisis, can be dated from August 9, 2007, when BNP Paribas terminated withdrawals from three hedge funds citing "a complete evaporation of liquidity".
answer:

BNP Paribas terminated withdrawals from three hedge funds