I know that in a double-slit experiment, the diffraction of the individual slits produces the envelope of the intensity pattern whereas the interference between the waves coming from the slits produces those little wiggles within that envelope. But what exactly is the difference between diffraction and interference other than how they affect the intensity pattern? Interference make sense to me because, physically, it is just applying the principle of superposition and adding the waves up. But I can't imagine a similar physical picture for diffraction. Also, how is it possible, in principle, to have interference without diffraction (in the context of slits)?