Absolute values of entropy are determined on the basis of the third law of thermodynamics, which is discussed later. Engineers are usually concerned with the changes in entropy. Therefore, the entropy of a substance can be assigned a zero value at some arbitrarily selected reference state, and the entropy values at other states can be determined from Eq.4-6 by choosing state 1 to be the reference state (S = 0) and state 2 to be the state at which entropy is to be determined. So, what is entropy? To answer this, let us ask: what is energy? The point is, do we exactly know what energy is?