Probability is just as relevant in psychiatry as it is in medicine; the great difference lies in its significance.

Through reference to Immanuel Kant's knowledge, this significance, and the ensuing consequences, are discussed in view of scientific psychiatric studies and pending revision of the psychiatric categories of the ICD-10 for the ICD-11 classification.

Most medical diagnoses are based on signs and symptoms.

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

Psychiatric diagnoses provide the foundation for scientific study and lead to philosophical probability, which Immanuel Kant called “philosophische Wahrscheinlichkeit” in German.

Medical Diagnoses, being based on objective signs, which refer to real objects, objects in an absolute sense, lead instead to mathematical probability, “mathematische Wahrscheinlichkeit” in German.

The difference between philosophical and mathematical probability will be shown through reference to Kant's works.

Knowledge based on mathematical probability commands greater authority than knowledge based on philosophical probability, as a comparison of the value ot two studies would demonstrate: one study naming the diagnosis luetic encephalitis, based on objective signs, and one study with the former diagnosis progressive paralysis, based on phenomena and symptoms.

This knowledge difference reflects, for example, continuing psychiatric research to find physical parameters for the mental disorder we call schizophrenia.