The two dimensional Ising model reads $$E[\sigma]=-J\displaystyle\sum_{<ij>}\sigma_i\sigma_j-h\displaystyle\sum_i\sigma_i,$$

where $E$ is the energy, $\sigma_i$ is the spin at lattice position $i$, each taking the value $1$ or $-1$ and $J$, $h$ are coupling constants. Now there is a statement (see Conformal Field Theory by Di Francesco etc., pp.63) that the case $J>0$ is equivalent to that of $J<0$ when $h=0$. How to understand that?

The two dimensional square lattice Ising model reads $$E[\sigma]=-J\displaystyle\sum_{<ij>}\sigma_i\sigma_j-h\displaystyle\sum_i\sigma_i,$$

where $E$ is the energy, $\sigma_i$ is the spin at lattice position $i$, each taking the value $1$ or $-1$ and $J$, $h$ are coupling constants. Now there is a statement (see Conformal Field Theory by Di Francesco etc., pp.63) that the case $J>0$ is equivalent to that of $J<0$ when $h=0$. How to understand that?

The two dimensional Ising model reads $$E[\sigma]=-J\displaystyle\sum_{<ij>}\sigma_i\sigma_j-h\displaystyle\sum_i\sigma_i.$$

Now there is a statement that the case $J>0$ is equivalent to that of $J<0$ when $h=0$. How to understand that?

The two dimensional Ising model reads $$E[\sigma]=-J\displaystyle\sum_{<ij>}\sigma_i\sigma_j-h\displaystyle\sum_i\sigma_i,$$

where $E$ is the energy, $\sigma_i$ is the spin at lattice position $i$, each taking the value $1$ or $-1$ and $J$, $h$ are coupling constants. Now there is a statement (see Conformal Field Theory by Di Francesco etc., pp.63) that the case $J>0$ is equivalent to that of $J<0$ when $h=0$. How to understand that?