A definition of a cubic space that is based on the idea of 6 equal sides is useful for mathematicians but restrictive for architects. Imagine instead that you define a cube as a space determined by six different conditions. The top and bottom are easily distinguished: one is a ceiling and the other a floor. As such their roles, materials and effects are considerably different. The remaining four sides can also be differentiated if you consider something as straightforward as sun orientation. Put into a context the four vertical sides of a cube would have four distinct relationships to light. One of your sides might contain an entry, thus rendering one side a ‘front’, one a ‘back’ and the other two as ‘sides’. If in your particular context the side on the left has a space outside of it that is different to the one on the right then these sides can start to take on different characteristics.
From this point of view a cubic space and its resulting characteristics is really something that emerges from the specific conditions that define the perimeter. Put another way, you should never conceive of spaces as abstract cubes, blocks or hermetic rooms. Every space that exists in the world has an orientation. Therefore, it requires more energy and explanation to conceive of and defend a space that is undifferentiated. It should be normal, rather than exceptional, that spaces consist of defined boundaries that differ, rather than consist of walls with holes in them.
How does this help?
This should help you consider all spaces as being determined by their location and relation to other spaces rather than conceived as isolated or abstract volumes. The cube in this discussion is nothing more than a stand-in for any space that you may be designing.
Source: There isn’t a precise source for this idea, but it is exemplified in the way Le Corbusier designed his spaces.
The two examples shown here happen to be cubic spaces and were chosen because they are clear examples of the principle. As noted above, this principle is not really about cubic spaces but about the way any space can be defined as a set of conditions rather than walls.