The soul is always in light, can never be in anything but light because the soul is light. Mind makes accommodation for both light and darkness - light from heaven, darkness from earth. When mind draws its content from earth and picks up shadow-thoughts and shadow-words, instead of the light-substance embodied in those thoughts and words, it cannot have surety.
For not only is shadow not light, but the tendency of shadow is to change its shape, to become smaller or larger, dimmer or deeper as light approaches or recedes. Under such circumstances the mind can never be too sure of this earth born intellectual mind-gathering called knowledge, which is nothing but the collection of names and forms and not true knowledge.