Have you ever lain awake at night fretting over how we can be sure of the reality of the external world? Perhaps we are in fact disembodied brains, floating in vats at the whim of some deranged puppet-master? If so, you are not alone - and what's more, you are in exalted company. For this question and other ones like it have been the stuff of philosophical rumination from Plato to Popper. In a series of accessible and engagingly written essays, 50 Philosophy Ideas You Really Need to Know introduces and explains the problems of knowledge, consciousness, identity, ethics, belief, justice and aesthetics that have engaged the attention of thinkers from the era of the ancient Greeks to the present day. Contents include: The brain in a vat, Plato's cave, Cogito ergo sum, The mind-body problem, The boo/hurrah theory, Ends and means, The categorical imperative, Acts and omissions, Do animals have rights?, The gambler's fallacy, Paradigm shifts, Occam's razor, Positive and negative freedom, Theories of punishment and Just war.