That we are morally responsible for what we do willingly and knowingly is a commonplace. That our moral responsibility extends as far as to cover at least the intended consequences of our voluntary actions and perhaps also the ones we did not intend, but could or did foresee, is equally beyond dispute. But what about omissions? Are we, or can we be, (equally) morally responsible for the harm that has occured because we did not prevent it, even though we could (...) have done so? Say, for all the enormous suffering, caused daily by famine, deprivation and curable diseases in the Third World countries? Moral intuitions and practices that one could consult in this matter seem to leave us in the dark. We regularly ascribe responsibility to people for harms resulting from their negligence or failure to fulfill professional duties. On the other hand, we tend to think that unless there is some evidence of the causal contribution that agents made to a harmful event and/or state, it is not really fair to blame it on them. And finally, to complicate things even more, most of us deny that omissions could effect anything (any change) in the world and consequently regard them as causally impotent (as well as possibly harmless). (shrink)

This is a review of those key thought experiments in physics from the late 19th century onward that seem to have played a particular role in the process of the discovery or advancement of theory. Among others the paper discusses Maxwell's demon, several of Einstein's thought experiments in relativity, Heisenberg's microscope, the Einstein-Schrödinger cat, and the EPR thought experiment.