As with so many theories of architecture that are presented as a history of the dicipline, I find Banham's opening pages in his book 'Architecture of the well-tempered environment', offers the most convincing for the 'act' of architecture.
"Mankind can exist, unassisted, on practically all those parts of the earth that are at present inhabited, except for the most arid and the most cold. The operative word here is 'exist' ... in order to flourish ... mankind needs more ease and leisure ... . A large part of that ease and leisure comes from the deployment of technical resources an social organisations, in order to control the immediate environment."
Reyner Banham
I rememer once that my lecturer Nik Aziz, told me once that the first act of architecture has always and will always be about conditioning an environment. An environment that will support and suggest how human will live and perceive its surroundings.