Are there any problems concerned with this theory, too? Of course. The obvious problem here is that without some kind of reinforcement, the deviation might go back and forth and cancel out any change. Then the Neogrammarians patched this theory by adding reasons for reinforcing the deviation such as simplification of sounds, or children imperfectly learning the speech of their parents. Simplification of sounds? Can you explain it a little further? Sure. The simplification of sounds basically states that certain sounds are easier to pronounce than others, so the natural tendency of the speakers is to modify the hard-to-say sounds to easier ones. An example would be the English word "nuclear", which man people pronounce as "nuclear". But who knows which one is more difficult to pronounce, "nuclear" or "nucular"? Exactly. Different people may give different answers. Simplification no doubt exists, but using it as a reason of language change is too subjective to be scientific.