= Solution
Choose the outward <unit normal> $n$ to the <boundary> of the chosen current material subbody. The <traction> $t(x,n)$ is the contact <force> exerted on that subbody per unit current <area> at the <boundary> point. Reversing the subbody side reverses $n$ and $t$.
By <Cauchy's stress theorem>, the dependence on $n$ is linear. With the usual column-vector convention the <Cauchy stress tensor> satisfies
$$
\boxed{t(x,n)=\tau(x)n,\qquad t_i=\tau_{ij}n_j.}
$$
The same point has different <traction> vectors on differently oriented planes, but a single <Cauchy stress tensor> encodes all of them. In the absence of body couples, <conservation of angular momentum> makes $\tau$ symmetric.
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