kant and wittgenstein on logically private languages and madness

In his Anthropology Kant offers:

The only universal characteristic of madness is the loss of common sense (sensus communis) and its replacement with logical private sense (sensus privatus)… For it is a subjectively necessary touchstone of the correctness of our judgments generally, and consequently also of the soundness of our understanding, that we also restrain our understanding by the understanding of others, instead of isolating ourselves with our own understanding and judging publicly with our private representations, so to speak. Thus the prohibition of books that advance only theoretical opinions (especially when they have no influence at all on legal commissions and omissions) offends humanity. For we are thereby robbed, not of the only, but still of the greatest and most useful means of correcting our own thoughts, which happens due to the fact that we advance them in public in order to see whether they also agree with the understanding of others; for otherwise something merely subjective (for instance, habit or inclination) would easily be taken for something objective. This is precisely what the illusion consists in that is said to deceive us, or rather by means of which we are misled to deceive ourselves in the application of a rule. – He who pays no attention at all to this touchstone, but gets it into his head to recognise private sense as already valid apart from or even in opposition to common sense, is abandoned to a play of thoughts in which he sees, acts, and judges, not in a common world, but rather in his own world (as in dreaming).

Compare what Wittgenstein says about a 'logically private language' in his Investigations:

Let us imagine the following case. I want to keep a diary about the recurrence of a certain sensation. To this end I associate it with the sign "S" and write this sign in a calendar for every day on which I have the sensation. -I will remark first of all that a definition of the sign cannot be formulated. -But still I can give myself a kind of ostensive definition. -How? Can I point to the sensation? Not in the ordinary sense. But I speak, or write the sign down, and at the same time I concentrate my attention on the sensation -and so, as it were, point to it inwardly. -But what is this ceremony for? For that is all it seems to be! A definition surely serves to establish the meaning of a sign. -Well, that is done precisely by the concentrating of my attention; for in this way I impress on myself the connexion between the sign and the sensation. -But "I impress it on myself" can only mean: this process brings it about that I remember the connexion right in the future. But in the present case I have no criterion of correctness. One would like to say: whatever is going to seem right to me is right. And that only means that here we can't talk about 'right'.

The commonalities are obvious. But Wittgenstein's is the deeper argument. For Kant, despite his 'so to speak', does rather leave one with at least the impression that there somehow could yet be such a thing as a 'logical private sense', a 'private representation' - rather than a mere illusion of one - whereas Wittgenstein helps us work through and relinquish the fantasy of the very idea of a 'sensus privatus'. He helps us see, that is, that we didn't actually know what we were doing and saying when we took ourselves to be reaching toward certain inner phenomena with such phrases.

Notwithstanding, whilst Wittgenstein's argument can helpfully be read as his working through the narcissistic tendency to mistake pseudo-contentful fantasies for contentful thoughts, and whilst psychoanalysis has more recently shown us the centrality of narcissism for psychopathology in general (and not simply in, say, narcissistic personality disorder), it seems it was Kant who first proposed that narcissistic illusions of sense are a 'universal characteristic of madness'.