# Why can't we have half-reactions by themselves?

Half reactions are said to have cell potentials that... well, are potential differences. You can't have a difference between $x$ and nothing else. You need a $y$.
or some other reference electrode. The SHE is quite common, and it was declared to have an arbitrary absolute cell potential of $\text{0 V}$; it is easier to be in reference to a voltage of $\text{0 V}$ than an absolute cell potential of $\text{4.44 V}$.