Hi: If we want to work out the expectation of: <0|T(φ1φ2)|0> ie. <0|<0|T(φ1φ2)|0>|0> apparently it is acceptable to pull out the <0|T(φ1φ2)|0>: So <0|<0|T(φ1φ2)|0>|0>=<0|T(φ1φ2)|0><0|I|0> I do realise this is a really stupid question, but I want to be 100% sure. Is this simply because an expectation is always a constant, not an operator which acts on a state? Can you always pull out an expectation in this way? Thanks.