Kraepelin (in Madness Explained)
This quote appears in Madness Explained by Bentall.
“Judging from our experience in internal medicine it is a fair assumption that similar disease processes will produce identical symptom pictures, identical pathological anatomy and an identical aetiology. If, therefore, we possessed a comprehensive knowledge of any of these three fields---pathological anatomy, symptomatology or aetiology---we would at once have a uniform and standard classification of mental diseases. A similar comprehensive knowledge of either of the other two fields would give us not just as uniform and standard classifications, but all of these classifications would exactly coincide.” —
Basically this is completely wrong, yet psychiatry in its medical form essentially depends critically on such assumptions. The idea that many distinct problems can yield similar symptoms, or that similar problems can yield very different symptoms, just gets silently assumed away. (For a technological example of the former: consider that there are many ways that a laptop can show a BSOD, the BSOD being an example of 'similar symptoms', and yet there isn't a single reason why a machine crashes with a BSOD. For an example of the latter, consider flipping a single bit in memory: similar problems are flipping other bits, but the consequences of a bit in memory being flipped can be quite varied.)