There's a common misunderstanding of the concept of emptiness in Buddhism - namely that "Buddhists believe that things don't really exist or Buddhists believe that nothing exists".
Emptiness really means empty of inherent existence. In other words, nothing contains the reason for its existence within itself. All functioning things or phenomena arise (come about due to...) from ever-changing relationships with other phenomena, including the minds of the observers. In my mind that means that nothing has any meaning in and of itself, things only have the meaning that we give them. Things have no meaning by themselves - either we collectively agree to call a box a box, or we individually assign meaning to something, like a grandmother's music box.
The teachings on emptiness are concerned with how things exist, not if and whether things exist (like planes, trains and automobiles) or why things exist (because God, the Devil or John Deere made them).
Language influences our thought. Let's consider how we use the words "exist, existed and existence". Maybe existence itself is an arbitrary concept. Maybe existence is a convenient,conventional truth.
We don't normally say that an explosion exists or existed (though there's no logical reason not to say that). And we don't normally say that the universe occurs. Yet an explosion and the expanding universe are similar entities, just operating on different timescales. From our point of view an explosion is a transitory event, but the universe is permanent. An explosion happens, the universe exists.
We don't normally say that an eclipse exists, and we don't normally say that Stonehenge happens. Yet both phenomena are the temporary coming together of masses in a geometrical configuration. In one case the sun, the earth and the moon in a straight line and in the other case, stones arranged in a circular pattern.
Relative to our lifetime, an eclipse is a temporary phenomenon, whereas Stonehenge is more permanent - built around 3,000 BCE. But there is no absolute distinction between phenomena that exist and phenomena that occur or happen. The distinction is arbitrary, based on the following considerations:
a.) The universe consists of billions and billions of particles in a constant state of movement.
b.) These particles form aggregates which hang together for a time and then disintegrate.
c.) Aggregates that hang together for more than a substantial fraction of a human lifetime (like a car) exist. Aggregates that hang together for a tiny fraction of a human lifetime (like a flash of lightning) 'happen' or 'occur'.
Yet, although I think my car exists, it is actually a series of events. The car I arrived home in tonight is not the car I set out in this morning. It has rusted a bit. The gears are worn a little more. Its tires are slightly less legal.
There's a Buddhist concept of subtle impermanence which states that nothing whatsoever remains identical from one moment to the next. So to say that something exists is ultimately an arbitrary statement. All we are saying is that its rate of disintegration is negligible on the timescale of our lifetime. In reality, all functioning phenomena are impermanent - it's just that some are seem impermanent than others.
All things are impermanent, and so all things are in reality processes. Things do not stay the same from one millisecond to the next. Anything composed of atoms is composed of parts in a constant state of flux. Existence is merely impermanence viewed in slow-motion.
Boxiness and Trayfulness
To find the true and unchanging essence of a box you would have to find the unchanging essence of a process, which is a logical contradiction. The nearest thing to finding a "box-essence" would be to say that these pieces of wood in this configuration perform the functions of a box. But that recognition comes entirely from your own mind (or from the collective minds of box-users).
If we were to cut the sides of a box off, it would perform the functions of a tray. If I say "I'll get a box to put this stuff in", then most people will understand that I'm going to fetch a container which performs the conventional function of a box, i.e. holds things. To do this it must have a bottom and four sides. A lid is optional.
The box exists from causes and conditions - the box-maker, the wood from which it is made, the trees, lumberjacks, sunlight, acorn, soil, rain, etc. A box exists dependent on its parts - bottom and four sides. The parts exist dependently on the box, otherwise they'd just be flat boards of wood. The box also exists because we decided to call it a box, not because of some inherent `boxiness' that all boxes have as a defining essence.
If it were a big cardboard box, and I cut a large section out of one side, then I could turn it upside down and it would be a kids play house. If I cut the sides of a wooden box down a inch at a time, then the box would get shallower and shallower. At some point the box would cease to exist and the tray would begin to exist. Or maybe the essence of `boxiness' would miraculously disappear and `trayfulness' would jump in.
Where does box end and tray start? I don't know. Maybe there's an building code that forbids the construction of boxes with insufficiently high sides, or maybe there's a Tray Descriptions Act. But whichever way, as well as existing in dependence on its parts, and on its causes and conditions, the box exists in dependence upon our minds... or the collective minds of the building code inspectors imputing a box over a certain collection of parts.
