Explanation:

If both energy and work were different from each other, then for the generation of every quantum of work there would not have been a corresponding disappearance of one quantum of energy in the body. For example, suppose one calorie of energy disappears, when you walk one mile. If you have walked two miles, two calories of energy must disappear correspondingly. This means, that only the energy is converted into work; not any other item like the instrument. By walking two miles the instrumental legs do not disappear partially by two quanta (like two calories). Hence, the instrument is not involved in the conversion of energy to work. Therefore, work is only another form of energy.