Most medical diagnoses are based on symptoms and signs.

Psychiatric diagnoses are based on symptoms and phenomena, as are certain other medical diagnoses.

With reference to Immanuel Kant’s Critique of Pure Reason, it can be shown that symptoms and phenomena are ideal objects, which we conceive as terms in our mind.

In contrast, physical signs refer to real objects, to objects in an absolute sense. Therefore, most medical diagnoses can be determined objectively on a physical basis.

Ideal objects, such as psychopathological phenomena and symptoms, are mental schemes (schemes of terms in the mind) which do not refer to real objects and, consequently, we cannot prove them on a physical basis, only carefully weigh them in our mind and ponder the presence of a phenomenon (symptom).

This is the major difference between objective medical diagnoses, on the one hand, and psychiatric diagnoses and medical diagnoses that cannot be proven on a physical basis, on the other.

This basis of psychiatric knowledge needs to be acknowledged and taken into consideration both in psychiatric practice and in science.

Ensuing consequences are discussed through reference to quotations from Immanuel Kant’s works and to Wilhelm Griesinger, who was aware of the symptomatological basis of psychiatric diagnoses.