The first in time I shall call Preparation , the stage during which the problem was 'investigated ...in all directions'; the second is the stage during which he was not consciously thinking about the problem, which I shall call Incubation; the third, consisting of the appearance of the 'happy idea' together with the psychological events which immediately preceded and accompanied that appearance, I shall call Illumination . And I shall add a fourth stage, of Verification ... Solis Press are pleased to be able to republish Wallas' seminal book on creativity that had sadly been out of print for many years. This edition is based on the first edition of 1926 and has been completely reset in fresh type.