Given the question: Information:  - A symptom (from Greek , "accident, misfortune, that which befalls", from , "I befall", from - "together, with" and , "I fall") is a departure from normal function or feeling which is noticed by a patient, reflecting the presence of an unusual state, or of a disease. A symptom is subjective, observed by the patient, and cannot be measured directly, whereas a sign is objectively observable by others. For example, paresthesia is a symptom (only the person experiencing it can directly observe their own tingling feeling), whereas erythema is a sign (anyone can confirm that the skin is redder than usual). Symptoms and signs are often nonspecific, but often combinations of them are at least suggestive of certain diagnoses, helping to narrow down what may be wrong. In other cases they are specific even to the point of being pathognomonic.  - Primary polydipsia or psychogenic polydipsia is a special form of polydipsia . It is usually associated with a patient 's increasing fluid intake due to the sensation of having a dry mouth . When the term `` psychogenic polydipsia '' is used , it implies that the condition is caused by mental disorders . However , the dry mouth is often due to phenothiazine medications used in some mental disorders , rather than the underlying condition . Some forms of primary polydipsia are explicitly characterized as nonpsychogenic .  - Thirst is the craving for fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids or an increase in the concentration of certain osmolites, such as salt. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, the brain signals thirst.  - Polydipsia is excessive thirst or excess drinking. The word derives from the Greek "very thirsty", which is derived from ("much, many") + ("thirst"). Polydipsia is a nonspecific symptom in various medical disorders. It also occurs as an abnormal behaviour in some non-human animals, such as in birds.    What is the relationship between 'primary polydipsia' and 'disease'?
The answer is:
instance of