What is a door?

We have an immediate image of a flat, vertical solid, swinging on hinges - which, unlike a gate, completes an encapsulation such as a room or a house. But just as the wall surrounding a doorway may limit passing the threshhold any other way, a gate limits something by its height - a limit not of a solid, physical kind, but rather a limitation of the entrant. A dog or horse, for example, cannot easily climb over the gate, though they may escape gravity for long enough to cross the threshhold by jumping. But if this were a common habit, soon the gate and fence will be rebuilt, only much higher.

Why, then, do short gates succeed? For this we need to examine the meaning of doorway a little closer...

A doorway is a portal between two STEMs (Space, Time, Energy and Matter intersections/encapsulations). A room is an enclosed STEM of virtually empty space, to which the temperature or mass of the space is irrelevant to the definition. One room may be separated from another based on temperature, such as a cooler, requiring something to prevent heat loss. Without a portal to the icebox, a fridge would have no use to us, so we place a small door on it which remains closed at all other times but when things are being added or removed.

It is very nice to know what one has in the freezer. Otherwise we are forced to open the door, taking more of our time and causing the electric meter to spin out of control. It is tough to remember what we have on hand - especially if we have kids. Now think how tough this would be if you had a thousand kids...

Introducing the supermarket freezer section - home of the discretionary portal.

You'll notice two basic types of freezers at the supermarket: The haves and the have-nots - those that have a door and those that don't. The doors are not like the kind one finds on their home freezers - they are more like a cross between a door and a window: See through portals. Only one problem - though they hold in the cold air, one must have a free hand to open the door.

The second type, lacking doors, provide shoppers with direct access to the frozen product - at a price: The dreaded heat loss. To compensate, though they may have eye-level shelves, they also have a trough shape below, which catches and holds the cold air, preventing it from escape. Like the dogs they are, cold air molecules are kept from escape by the use of a "gate". This discretionary mechanism allows the free passage of solids while controlling cold air loss to some extent.

The glass doors of the other type of freezer hold in the cold air far better (keep the heat out, technically). Although the passage of solids is inhibited, however, the passage of light is not. This is critical to sales, as we all know from the phrase "Out of sight, out of mind".

Let's look at this again...

1) Home freezer: Maximized control of cold air loss through use of a solid portal. This material is opaque, as it happens to be a cheap means of insulation. This is a fully discretionary portal, allowing nothing in or out of the STEM, unless the portal is open.

2) Glass-door freezer: Moderate control of cold air loss through use of a solid portal. The material is transparent, to allow passage of light, allowing one to see the contents of the STEM before entering. This is a semi-discretionary portal, allowing light out at all times, but cold and material are trapped until the portal is opened.

3) Open freezer: Limited control of cold air loss achieved through the use of gravity's effect on cold air. The portal material is virtually nonexistent, being air, allowing the free movement of light and matter, while restricting the leakage of the cold air. This is a non-discretionary portal, relative to the key functions of visibility and accessibility, while being somewhat constant in its cold air loss. The cold air behaving close to a clear liquid, one may picture products in this freezer as coins in a fountain, where one can just reach in and get the coin, after spotting it in the water (and checking for security guards).

A dog is held in the open freezer of ones yard: Gravity holds the dog down to low levels, as if cold air, allowing the low gate to contain it. Though, with force, the dog may "splash out", under ordinary circumstances this will not happen. When cold air "splashes out" from an open freezer, frost will form - a telltale scar of cold-air-STEM-portal inefficiency.

A gate and a screen door are quite similar, allowing the free passage of air, but limiting the passage of solids. These two portals also swing. This is but one action the solid portal can take.

(To be expanded...)