Glenn introduces concepts from medieval epistemology (i.e. the branch of philosophy dealing with knowledge and truth) and how these got challenged in the sixteenth and seventeenth century, particularly through the recovery of the ideas of the ancient Greek skeptic Pyrrho. After a foray into Descartes, who tried to answer Pyrrho, we look at Blaise Pascal, the father of probability theory. Pascal used a barometer to short-circuit Pyrrho’s approach and in the process laid the foundation for a new approach to knowledge based on probability. This new epistemology has shaped thinking in the West ever since and largely created the modern world.