  Witten's approach to Khovanov homology of knots is based on the
five-dimensional system of partial differential equations, which we call
Haydys-Witten equations. We argue for a one-to-one correspondence between its
solutions and solutions of the seven-dimensional system of equations. The
latter can be formulated on any G2 holonomy manifold and is a close cousin of
the monopole equation of Bogomolny. Octonions play the central role in our
view, in which both the seven-dimensional equations and the Haydys-Witten
equations appear as reductions of the eight-dimensional Spin(7) instanton
equation.
