Following this philosophy means that you will never end up with a room that feels too matchy-matchy or unlived-in. It almost guarantees that it will feel warm and welcoming. The 'something old' could be a piece of vintage or antique furniture that may be showing a few signs of its long life; 'something new' may be a modern couch or lamp; 'something rough' could be a rug, a textured cushion, or a woven basket; and 'something smooth' could be as simple as plastered walls or polished floorboards, or even a mirror.
The underlying reason that this philosophy works so well is simply that it conforms to one of the 'principles of design' - contrast. (As it happens, contrast is my favourite principle!) Jason is describing a contrast between old and new, and between rough and smooth.
Here's a good example:
Image from Skona Hem
This image combines a vintage (or vintage-style) table (the old); a clean-lined modern lamp (the new); a large woven basket and a visually textured floor (the rough); and a stunning teal wall (the smooth). Although all the elements are quite disparate they come together beautifully as a unique expression of the owner's taste. And as well as the contrast, the similarity between the pattern on the basket and the pattern in the parquetry flooring is a subtle example of another design principle, harmony.
I aim to follow the 'something old, something new, something rough, something smooth' philosophy in my shop as well as at home . . . This is why I started stocking small pieces of vintage furniture such as cabinets and chests, as well as glassware and ceramics. Although the majority of the items I stock are modern, mixing in some vintage pieces really softens the effect, as well as giving me a chance to show my customers ways of integrating pieces they may already have into a contemporary home. I really love the warmth that a few vintage items bring to a room.