Yesterday, I was washing baby bottles, an experience I've had many hundreds of times over the last few years and will have many hundreds of times more in the coming year plus. When washing a bottle, particularly the nipple, special care must be taken to clean the inside of the bottle and nipple. The inside of the nipple, especially, gets cloudy and, if left for any length of time, formula will dry up and clump up in the tip of the nipple. We use a dishwasher to clean our dishes, but that won't suffice with bottles. We need to scrub the inside surface of the nipple and bottle prior to putting them in the dishwasher. It's hard work that turns washing dishes from a 5 minute chore into a 20 to 30 minute task.
Of necessity, then, my wife and I focus our efforts on the inside of the bottle and nipple when cleaning. Yet, if we only washed the inside of the bottle, but left the outside alone, the bottle itself would not be considered clean. As it so happens, the dishwasher takes care of the outside of the bottle and nipple, but if we were not to perform that exterior cleansing, the whole thing would, in fact, be dirty.
I think, actually, that that is one of the essential points of the book of James. Whereas Jesus preached about those who clean the outside of the cup, but leave the inside filthy, James gets at those who think they can only clean the inside but leave the outside dirty. In fact, both need cleansing or else the end product is soiled. Let us, then, not neglect either, but purify ourselves completely by His Spirit.