‘Empiricism’ stresses that, for Aristotle, knowledge or science ultimately derives from perception. That is the source of the concepts in terms of which we seek to understand reality. Knowledge is not the same as perception, but is bred by generalization out of perception. This raises questions. Is sense perception reliable? How can we tell? How can we distinguish illusion from genuine perception? Are we justified in moving from particular observations to general truths? How can we know if our observations are sufficiently numerous or representative? Aristotle is dismissive of the sceptics in the Metaphysics: he thinks their views are not seriously held and need not be taken seriously.