I've just finished reading a thread (what is an inertial reference frame), where the participants are arguing over whether or not acceleration is relative. I think this indicates some disagreement over the meaning of 'absolute' and 'relative' which philosophical terms are thrown around entirely too much. In order to conclude that 'acceleration is relative' one has to be using some kind of meaning of the term 'relative'. I suspect the disagreement in the thread I was reading, is because the notions of 'absolute' and 'relative' are somewhat hazy. My position is this. Velocity is relative. Thus, it would be wrong to say that something is moving at 5 miles an hour. You need to say that something is moving at 5 miles per hour relative to me, and you are taking yourself to be at rest. Conversely, the other thing says that you are moving 5 miles per hour and it is at rest. Now, acceleration is going to be defined as dv/dt. So, the question now is, must the derivative of a relative quantity, also be relative, for solely mathematical (kinematic) reasons alone, which have nothing to do with dynamics? My answer is yes, and there is a great deal of reasoning behind it if anyone is interested. I would be interested in hearing others thoughts on the matter.