The law that the just noticeable difference between two stimuli is a constant ratio of the two stimuli.

This means that if you want to get something done, start with a very minimum effort and just increase it very slowly, this is going to help you build habit and lead you to your desired goal.

This can be used for sales by slowly getting inside your prospect's personal space by interacting in a “non-sales” way to eventually get them to interact with you.