Theorem 7.7:

The set $\pi_1(X, x_0)$ of homotopy classes of loops in $X$ based at $x_0$ is a group with respect to the binary operation defined by (7.1). The unit element of the group is the homotopy class of the constant loop at the base point $x_0$ and the inverse of $[\gamma]$ is the homotopy class of the loop $\gamma^{-1}$.