In episode 61, when the fight goes on between Edward and Pride, Kimblee interferes with the mind of Pride. Edward uses this moment to attack Pride by getting into his mind (or body maybe). However, as this occurs, the dialog of Pride to Edward is this:
he turned himself into a philosopher's stone to infiltrate me?
What does this mean and how it is possible as Edward is not in possession of a philosopher's stone (not sure whether he can do it with a philosopher's stone)?
dimwittedanimal
provided the best response. When Darius and Heinkel hold Edward still (and remove the pipe in his chest), Edward notes "this'll take a few years off my life": he's aware he's performing medical alchemy and using his soul as a battery/exchange-for-parts to help him repair his body. Medical alchemy seems to violate the laws of thermodynamics less than other forms of alchemy (i.e. mass and souls obey equivalent exchange, but energy is free, except when repairing organs). Seems in human transmutation & medical alchemy, the whole really is greater than the sum of the parts.