measurement

Using a camera, emo and ero could be defined as the difference between two images taken shortly after each other. Differing pixels would be emo, same pixels ero. For example, a ball that rolls down a slope would itself not be emo as a physical object, but emo would be the area the ball spawns between the two images (excluding the middle if the ball is uniformly colored).

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A camera can only register ero and emo, and thus only transitions ero↔emo, while transitions that would cross between in and out would not be part of the picture.

Measurements inside might be performed indirectly by measuring brain activity outside, or maybe by focussing on what is recurring inside and thus in a way stable inside, on maybe often abstract insights (eri) of a logical nature.

The most basic form of eri might be pairs of opposites, which could maybe be assembled to form more complex concepts, possibly inspired or guided by zodiacs and similar cultural concepts.

leads

• Even if a formal model of the elements defined in terms of in/out and rest/move and their transformations grew into a ‘scientific way of doing metaphysics’, as aimed at in Kant’s Prolegomena to Any Future Metaphysics That Will Be Able to Present Itself as a Science, it would essentially be air, something that rests inside the mind (eri). It would not be complete without also including the other three elements in some form, say, in performance art, or whatever.
• Moreover, it would likely not be possible to deduce the whole world from the definition of elements alone, at least doing so would likely be as hard as finding a theory of everything in modern science. Some additional, a priori unprovable assumptions might be necessary to synthesize the world.
• The concept of a “ball” is a priori much more complex than comparing two images, which becomes evident once you try to program computers to recognize (3-dimensional) items on 2-dimensional images. How a ball comes to be in the mind appears to require a lot of interaction with the environment (often quite early as a child), and in the end it is philosophically not so clear whether a “ball” is rather a natural thing, something that objectively exists, or instead rather a purely abstract cultural creation useful for interaction with others. See also Kant or Plato’s Allegory of the Cave.
  The above definition of emo↔ero appears thus fundamental, but is possibly already different from immediate experience of the world in which a rolling ball is never seen as two crescents. It reminds also of the shadows in Plato’s Cave, which even remind of the souls of the dead that dwell in Hades as shadows, as depicted in Homer’s Odyssey. In other words, the above definition might already project reality onto something in which crucial information might already be lost, or not.
• Could maybe only activity cross between in and out, but not elements ? Would activity travelling from in to out transform both eri to emi and ero to emo ? That would at least be consistent with a camera only recording ero and emo.
• In a harmonic oscillator, two kinds of energies are transformed into each other. For example, for a mass on a spring, the energy in the spring transforms into the kinetic energy of the moving mass and vice-versa. This gives the motion of the oscillator four special states, when either of the energies is extremal. And the motion between these states is periodic, thus overall reminding of the circle of elements.
  However, the natural pairing of extremal states of a harmonic oscillator is opposite states in the cycle, which naturally fits rest/move in the elemental circle, but makes it hard to relate two pairs of adjacent states to opposites like active/passive or in/out in a natural way.
• The four elements can be grouped into 3 different pairs with opposing attributes, including maybe these:

  rest/move	in/out	passive/active
  bind/release	wet/dry	cold/hot
  	soft/hard	heavy/light
  	malleable/brittle	inert/swift
  	mixed/isolated	dense/thin
  	collective/individual	dark/light
  		female/male
  		moon/sun
  		night/day
  		un-/conscious

  Some pairs on the right have a historically patriarchal touch, which however still partially reflects nature.