Have you ever laid 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.;The brain in a vat -- Plato's cave -- The veil of perception -- Cogito ergo sum -- Reason and experience -- The tripartite theory of knowledge -- The mind-body problem -- What is it like to be a bat? -- The Turing test -- The ship of Theseus -- Other minds -- Hume's guillotine -- One man's meat ... -- The divine command theory -- The boo/hoorah theory -- Ends and means -- The experience machine -- The categorical imperative -- The golden rule -- Acts and omissions -- Slippery slopes -- Beyond the call of duty -- Is it bad to be unlucky? -- Virtue ethics -- Do animals feel pain? -- Do animals have rights? -- Forms of argument -- The barber paradox -- The gambler's fallacy -- The sorites paradox -- The king of France is bald -- The beetle in the box -- Science and pseudoscience -- Paradigm shifts -- Occam's razor -- What is art? -- The intentional fallacy -- The argument from design -- The cosmological argument -- The ontological argument -- The problem of evil -- The freewill defence -- Faith and reason -- Positive and negative freedom -- The difference principle -- Leviathan -- The prisoner's dilemma -- Theories of punishment -- Lifeboat Earth -- Just war.