$$\Delta E_\mathrm{rev} = \Delta W_\mathrm{rev}$$ $$\Delta E_\mathrm{irrev} = \Delta W_\mathrm{irrev}$$ I got that if these both take me to the same point then $\Delta E$ will be same, but why must $W_\mathrm{rev}$ and $W_\mathrm{irrev}$ be different?