Bestellterminal in einem Restaurant
you order.
a tray appears.
what happened
in between?
image: ai-generated (gpt-image-2)

every computable piece has this shape

a box with clear edges brings its own test

if you know the input and expected output, you have a test.

the sender is a box

what enters the receiver?

your eyes see red. the box sees four numbers.

try it: what does the sensor see?

boxes connect when their edges agree

an agreement at every boundary.
to find the fault,
follow the data.
test one boundary at a time.

one box from the outside

open a box only as far as your question requires.

the shape survives every level

two relationships, two questions

both are ways of cutting a problem, as in cutting problems: into steps in a row, or into smaller work inside.
the right level is the one
you can test.

every box leaves two questions

the map of the module

an AI agent is a box, too

you do not need trust when you have a test.
Küche eines Fast-Food-Restaurants hinter der Theke, vorn ein Tablett mit Burger und Pommes
behind the counter:
inside the box
is a program.
image: ai-generated (gpt-image-2.5-flare)