When we talk about “lightness” we refer to the horse’s body, to the level of intensity with which we would like to give aids.
Softness, however, refers to the horse’s mind. The horse can be light on the aids and yet not be fully with you. If you also have his mind, then it is not only two bodies performing a movement but two minds in sync. And if you thought that your aids were light before, you will be surprised how light they can actually be!
Tell it to a mare…(sigh)