Why is it impossible to indicate the direction of movement of the boat in the fog when the banks

Why is it impossible to indicate the direction of movement of the boat in the fog when the banks of the river are not visible?

Mechanical movement is a change in the position of the body relative to other bodies.

The body, relative to which the movement is considered, is called the reference body.

When moving on a boat, the reference body is the coast. In the fog, the reference body is not visible, it is impossible to determine how the boat changes its position relative to the coast. Therefore, in the fog it is impossible to determine in which direction the boat is moving.