The phrase normally used is ‘theory and practice’ but Alexander wants to emphasize that, in his technique, practice precedes theory. It first appears in his Introductory to UCL, and several times in UCL, specifically stating that his theory ‘ﬂowed from’ the practical procedures:
My life-work has been one of dealing with practical procedures based on the principle of unity and with the associated theoretical conclusions which ﬂowed from them.
However, the preceding of ‘practice’ before ‘theory’ is a late phrasing.
‘Theory and practice’ was used in ‘The theory and practice of a new method of respiratory re-education’ (1907), in ‘Why we breathe incorrectly’ (1909), and in MSI’s ‘Preface to First Edition’ (1910), and in ‘New Preface’ (1918). Alexander discusses the bridging of the gap between theory and practice in the preface to the new edition (1946) of CCC.
Note that in UCL he talks about the ‘acceptance of the theory and practice of non-doing’.