Yes, it is frightening, especially since (I think) I am an "engineer,
married with adult children, own the house you are living in and the car in
the driveway, and so on."
That is a vivid description.
But even as I am being hauled away to the psychiatric ward, can I not
logically cling to at least one belief? According to Wikipedia, Rene
Descartes said, "But I have convinced myself that there is absolutely
nothing in the world, no sky, no earth, no minds, no bodies. Does it now
follow that I too do not exist? No: if I convinced myself of something [or
thought anything at all] then I certainly existed. But there is a deceiver
of supreme power and cunning who is deliberately and constantly deceiving
me. In that case I too undoubtedly exist, if he is deceiving me; and let him
deceive me as much as he can, he will never bring it about that I am nothing
so long as I think that I am something. So, after considering everything
very thoroughly, I must finally conclude that the proposition, I am, I
exist, is necessarily true whenever it is put forward by me or conceived in
my mind." (AT VII 25; CSM II 16–17)
"If I am sane, it is impossible to know for sure that I am sane."
Everybody believes he is sane, whether he is sane or not, and nobody can
prove he is sane. In psychiatry, this is the key problem with delusions. If
it were possible in general to prove one's own sanity, then deluded
patients, who more often than not retain their ability to think logically,
would be able to demonstrate to themselves that they were deluded. But by
definition of a delusion, this is impossible.
If you want to know what it is like for a psychotic patient to have forced
treatment, imagine that people from the local psychiatric facility knock on
your door tonight and, after interviewing you, politely explain that your
belief that you are an engineer, married with adult children, own the house
you are living in and the car in the driveway, and so on, is actually all a
systematised delusion. All the evidence you present to show you are sane is
dismissed as part of the delusion, and all the people you thought you could
trust explain that they agree with the psychiatric team. You are then
invited to start taking an antipsychotic drug which, over time, will rectify
your deranged brain chemistry so that you come to understand that your
current beliefs are delusional. If you refuse the medication, you will be
taken to the psychiatric ward with the help of police, if necessary, where
you will again be offered medication, perhaps in injection form if you
continue to refuse tablets.
Frightening, isn't it?