Buddhi is another constituent of Purusha. In terms of modern Psychology, Buddhi comprises intellect and will. It contemplates the circumstances that call for an action and provides rule of conduct. Will control the disposition, in harmony with the dictum from Buddhi. 

Manas has an ability to perceive various stimuli through Indriyas. Also, it can analyze them in terms of merits & demerits. Based on such analysis, Buddhi produces a decisive knowledge. Thus, Buddhi-the intellect is ultimate decision-maker.