Addey's most important contribution to modern astrology was the Harmonic theory, which sought to put the understanding of astrological effects on a clear and rational footing. Starting from the great Platonic statement (Timaeus, 37d) that "Time is an image of eternity flowing according to number", Addey identified astrology as "the study of effects in the world of flux and change" in a 1958 article, ‘The Search for a Scientific Starting Point’; and later articulated the fundamental law – "all astrological effects can be understood in terms of the harmonics of cosmic periods". In other words, the temporal world is only truly understood when it is seen as making manifest the great eternal ideas – Platonic Forms – in ordered cosmic periods.