Example: A person is standing on the top of a roof and dropping a feather. In calculating how long it takes for the feather to reach the ground, to make the maths simpler, one might make an assumption: that the effect of air resistance can be ignored. This assumption makes the problem simpler, but is unlikely to lead to a good prediction as to the time it will take for the feather to fall. Thus, making the assumption that air resistance can be ignored is in this case not the "simplest" in concept, but the simplest in other respects (in this case, the maths). "Not" making the assumption here "is" the "simplest" in concept because it involves making fewer assumptions.