If you wanted to define the theory of modern architecture at its purest you could do worse than "A Contemporary City of Three Million" from 1923 by Charles-
And what exactly was this theory? Sorry, but if you have to ask, then you are far too bourgeois to understand.
Okay, okay, I'll condescend this once by explaining that the important thing is not what it is, but what it isn't. And what it isn't is bourgeois.
Look at this city, for God's sake. The buildings are all straight lines and boxes. They stand perched on columns for no good reason. Nothing gives any impression of being handmade that can't be stamped out by a machine. Craftsmanship is nowhere to be found because only the bourgeoisie can afford craftsmen. Everything is concrete, glass, steel, and stucco. No colours are used when white, grey, black, or beige are available. Functionalism is everything, right down to the sheer facades. There are no eaves and the roof is inevitably flat. Never mind that flat roofs leak, are structurally weaker than peaked roofs, harder to construct, and tend to collapse under heavy loads of snow. Or that the lack of eaves mean that the rain is free to cascade down the sides of the buildings until the concrete becomes a mottled grey mess like fish fillets that have been left out on the doorstep overnight. At least it isn't bourgeois.
But what are these buildings supposed to be? Government offices? Headquarters for some super mega conglomerate?
Yes, Le Corbusier expected the masses to live in these giant concrete hives in flats stripped bare of any ornaments or colour and when they finally manage to escape from their warrens they have nothing to look forward to except empty spaces between the buildings and the the most dangerous aerodrome in history slapped smack in the middle of the mall just to confuse people and keep the neighbours awake.
Fortunately, nobody would touch Le Corbusier's project with a bargepole, so civilisation was spared a very expensive demolition bill decades later.