I refer to entropy.
It is still rather difficult to strip entirely this very important notion of all analytical adornment.

Many physicists hesitate to utilize it, and even look upon it with some distrust, because they see in it a purely mathematical function without any definite physical meaning.

Perhaps they are here unduly severe, since they often admit too easily the objective existence of quantities which they cannot define.

Thus, for instance, it is usual almost every day to speak of the heat possessed by a body.

Yet no body in reality possesses a definite quantity of heat even relatively to any initial state; since starting from this point of departure, the quantities of heat it may have gained or lost vary with the road taken and even with the means employed to follow it.