The traditional ideas of depression as an affective illness are examined and evidence is presented that contrary to the view presented in many texts, depression is also associated with thought disorders as severe as those experienced in other psychopathological illnesses. A model of depression as a single entity is proposed that suggests that the various manifestations of the disorder are due to a single underlying mechanism. This mechanism is conceived of in terms of cognitive style and cognitive control theory and evidence is presented that this is an adaptive mechanism for the individual and consists of selective blocking of incoming stimuli or in more general terms, 'rigidity'. Twelve depressed patients were examined using a battery of tests which yielded 22 measures including level of depression, rigidity, anxiety, field-dependence and various scales of personality. These measures were then correlated using Pearsons Product-Moment correlation coefficients and were then subjected to cluster analysis using Tryons modification of Holzinger and Harmon's B-coefficient technique. This gave four meaningful clusters of which three matched the components of Becks primary triad, with the important addition that this analysis reveals the importance of rigidity as a factor in Beck's second component-negative self-concept.