Upadana is fourfold: 1. Attachment to sensual pleasures; 2. Attachment to wrong and evil views; 3. Attachment to mere external observances, rites and rituals; and 4. Attachment to self, an erroneous lasting soul entity. Man entertains thoughts of craving, and in proportion as he fails to ignore them, they grow till they get intensified to the degree of tenacious clinging.