Nature’s balance defines wights. Too little connection to your Nature, and you lose yourself. You lose your connection with the world and go mad. Too much connection to your Nature, and it consumes you. You become an unthinking beast, ruled completely by your instincts. In between those two extremes, wights find their own thought and will. Nature can provide a pool of strength to call upon in distress, but at the same time, reliance on instinct makes it harder to learn new things or master new skills.
Different peoples have different Natures. For the mice, Nature helps in foraging, climbing, hiding and escaping. Mice can tap into their Nature to help themselves at other tasks, but when they do, they risk taxing their Nature. Taxed too far, and Nature begins to drop. Some members of the Mouse Guard have driven themselves so far that they go insane.