In a thought experiment, instead of bringing about a course of events, as in a normal experiment, we are invited to imagine one. We may then be able to ‘see’ that some result follows, or that some description is appropriate, or our inability to describe the situation may itself have some consequences. Thought experiments played a major role in the development of physics: for example, Galileo probably never dropped two balls of unequal weight from the leaning tower of Pisa, in order to refute the Aristotelian view that a heavy body falls faster than a lighter one. He merely asked us to imagine a heavy body made into the shape of a dumbbell, and the connecting rod gradually made thinner, until it is finally severed. The thing is one heavy body until the last moment, and then two light ones, but it is incredible that this final snip alters the velocity dramatically. Other famous examples include the Einstein-Podolsky-Rosen thought experiment. In the philosophy of personal identity, our apparent capacity to imagine ourselves surviving drastic changes of body, brain, and mind is a permanent source of difficulty (see split-brain phenomena, teleportation). There is no general consensus on the legitimate place of thought experiments, either to substitute for real experiment, or as a reliable device for discerning possibilities. Thought experiments one dislikes are sometimes called intuition pumps.