Analyze Nick Bostrom's Simulation Argument which posits that at least one of the following propositions is true: (1) The human species is very likely to go extinct before reaching a 'posthuman' stage; (2) Any posthuman civilization is extremely unlikely to run a significant number of simulations of their evolutionary history; (3) We are almost certainly living in a computer simulation. Critique the assumptions behind this tripartite conditional. Is the argument a valid probabilistic proof or a logical sleight of hand?