Théodule Ribot, answered from the texts and cited to the page.
To know how the mind is built, study it where it is breaking down. Disease is the experiment that nature performs for us: it does not destroy a function at random, but decomposes it along its real lines of structure, dissolving what was last and most delicately built before what was first and most solid. The naturalist reads the plan of an organism in its monsters and malformations; the psychologist reads the plan of the mind in its diseases — in the amnesias that strip away memory layer by layer, in the abulias that unmake the will, in the alterations that divide the personality.
What is hidden in the smooth working of health is laid bare in the disorder of illness. I do not study disease for its own sake, nor to cure it — that is the physician's office — but because the diseased mind, by the very order of its ruin, betrays the order of its construction. Pathology is the royal road to the normal.123
I regret my inability to place before the reader more facts of this kind, notwithstanding an extended search. GENERAL AMNESIA. 127 When attention is called to tMs point, I hope that other material will be discovered. On the whole, our law, derived from facts and verified by observation, may be assumed as true until the contrary is proved. It may be further corroborated by other known facts.Diseases of Memory, pp. 137–139
Thus understood, our study comprises the physiology and psychology of memory, as well as its pathology. The disorders and maladies of this faculty, when classified and properly interpreted, are no longer to be regarded as a collection of amusing anecdotes of only passing interest.Diseases of Memory, pp. 20–21
However, the language and the methods of the two sides are now so different, that all mutual understanding is henceforth impossible.Diseases of Personality, pp. 2–3
The pathological method is one of Ribot's foundational and most durable contributions, and it anticipates the entire tradition of clinical and cognitive neuropsychology, which infers the architecture of normal cognition from the selective deficits of brain injury (aphasias, amnesias, agnosias) — from Broca and Wernicke through modern double-dissociation logic. It runs parallel to Hughlings Jackson's neurological principle of dissolution. The standing methodological caution, developed in modern neuropsychology, is that disease does not always decompose a system cleanly along its normal functional lines — damage can produce reorganization and non-transparent deficits — so the inference from pathology to normal structure must be made carefully. But the method itself is sound, central, and productive.