"Dementia" today vs 100 years ago -- did it mean the same thing?

Question

I know that words for mental illnesses have changed quite a bit in the past century or so. Informally, I think most people see a difference between "crazy" and "unintelligent" today. "Dementia" is also understood to mean a general loss of mental ability: memory and cognition. But in the 1920s, schizophrenia was called Dementia Praecox. Did the medical term reflect a different understanding of schizophrenia, that it was a sort of what in modern times would indeed be called dementia or did it reflect a less strong distinction among medical professionals a century ago between "crazy" and "unintelligent." I think it is true that in schizophrenia there is indeed a cognitive decline but I do not think that is the primary characteristic and also I believe schizophrenia sometimes goes away where as what is today called dementia almost never reverses and in fact usually continues to get worse.

EDIT:

What got me going on this is I was reading The Making of the Atomic Bomb (Rhodes) in which Oppenheimer in his 20s was diagnosed/misdiagnosed with "Dementia Praecox" -- certainly there are few men less likely to have major cognitive deficits. He was in the middle of getting a doctorate in physics and he would end up managing the technical aspects of the Manhattan Project, etc. -- he was a well-known smart guy. So I remain confused about the term.

However, while J. Robert Oppenheimer certainly was a great genius, capable in many different areas, mathematics, physics and even the humanities, he did behave strangely in his 20s, attacking one friend and trying, literally, to poison another.

So to me he sounds "crazy" (in the normal sense of the word) but not suffering from cognitive decline.

Answer 1

Dementia was a mental illness 100 years ago also but it was a broader concept that time, so it doesn't mean the same thing today and it is better understood today as a disorder or syndrome. Dementia was first classified as a mental illness in 1806 per OED, and before that it meant insanity, madness as its etymology suggests: classical Latin dēmentia madness, insanity, craziness, folly < dēment- , dēmēns dement adj. + -ia -ia suffix¹. The words demented and the rare dement share the same etymological root. As provided in OED; here are the two definitions of dementia, the earliest citation from 1598 and the citation from 1806, and a brief historical note about how the word has changed in time:

1. Medicine. Originally: insanity, esp. when primarily affecting thinking and judgement; (also) absence or impairment of intellectual faculties. In later use: spec. impairment of memory and of abstract thinking, often with other disturbances of cognitive function and with personality change; a syndrome characterized by this, resulting from primary degenerative disease of the brain (most commonly Alzheimer's disease in the elderly), or from various other conditions (cerebrovascular disease, infections, tumours, etc.) which affect the brain.

More common in legal than in medical use until the early 19th cent., when démence appeared in French classifications of mental illness by Pinel and Esquirol (cf. quot. 1806). The definition of dementia in neurology and psychiatry has undergone many changes since this early use.
In popular and non-technical use now often identified with Alzheimer's disease.
presenile, senile, vascular dementia: see the first element. See also dementia praecox n.

1598 J. Mosan tr. C. Wirsung Praxis Medicinæ Vniuersalis i. xii. 130 (heading) Of Melancholia or Dementia, a woonderfull madnesse.
1806 D. Davis tr. P. Pinel Treat. Insanity 252 To cause periodical and curable mania to degenerate into dementia [Fr. démence] or idiotism.

2. gen. Complete loss of judgement; (wild) foolishness resembling insanity; an instance of this.

[1627 W. Sclater Briefe Expos. 2 Thess. 86 In his apprehension it sounds, putting beside their minde or right wits; as if some dementia should seize them when once they gaue way to vnsound doctrine.]

OED gives demency, now obsolete, as an earlier term from a1529:

Obsolete.

1. Madness; infatuation.

a1529 J. Skelton Why come ye nat to Courte (?1545) 679 The kynge his clemency Despenseth with his demensy.

2. Medicine. = dementia n. [translating French démence (Pinel).]

1858 J. Copland Dict. Pract. Med. II. 441/1 M. Pinel arranged mental diseases into—1^st, Mania..2^d, Melancholia..3^d, Demency, or a particular debility of the operations of the understanding, and of the acts of the will.