Alright, I have a kind of dumb question: Why do I distinguish between dq and dqi when considering the propagation from qi to q to qf? For example, if we want the wave function at some qf and tf given qi and ti, we may write: ψ(qf,tf)=∫K(qftf;qiti)ψ(qi,ti)dqi Why do we distinguish between dqi and say, dq, of an intermediate point q? I understand that a second integration arises, naturally, since another propagator is defined. I just don't get the need to distinguish? I feel like it's almost book-keeping, like we write out (or don't since we use product notation followed by a script D) our dq's so we don't have to write out more integrals... I know that's not right though, because we treat q and qi as different things... Some direction would be great...I feel kind of dumb for whatever I'm missing. edit: I think I mis-titled this a little, but it's close enough, I hope.