Is there a mental illness that almost completely stops you from thinking? I know the title seems insane, and this is most likely impossible, but is there a mental illness that almost stops a person from thinking? By thinking I mean the &quot;talking in your head&quot; thing. I'm not looking for a mental illness that makes them unconscious, but rather a mental illness that keeps them conscious and almost stops them from thinking. I have been saying this a few times, but I feel like it's false, since thinking is a person can't stop thinking if they are alive and conscious(right?). A more realistic question is: &quot;is there a mental illness that causes one's thinking abilities to severely decline?&quot;
Thinking is mainly supported by the frontal lobes (although the ARAS – the Ascending Reticular Activating System – is essential for its arousal and other cortical areas are essential for its sensory inputs and other associative functions, such as language, spatial orientation, and emotion, which are closely associated with thinking). So, anything that affects the frontal lobes can affect the thinking.
But frontal lobes never stops function if one is still conscious – neurons in the frontal lobes will keep firing as long as  there’s a stimulation from the ARAS. So, to literally stop thinking (or all neurons in the frontal lobes stop firing) in still-conscious people is impossible.
However, frontal lobes can be interfered to the degree that the thinking is significantly impaired or retarded and the person seems to stop thinking. This can happen in a varieties of conditions, such as:
Psychological: severe emotional overload (e.g., by a sudden loss of something so precious, an unexpected devastating event, or an overwhelming rage), schizophrenia in some stages (e.g. catatonic stage), severe depression
Pharmacological: overdosage of antipsychotic, sedative, or other psychoactive drugs
Pathological:

all metalbolic derangements (hypoglycemia, hyponatremia, hypercalcemia, etc.)
some medical conditions (hypothyroidism, hypocorticolism, hypothermia, etc.)
frontal lobes disorders: frontal lobe tumor, infarction, hemorrhage, encephalits, dementing diseases
other causes: post- cerebral contusion, post-ictal stage

At some stages of the above conditions, either before the patients lapse into coma or after they emerge from coma, the patients will appear dazed, muted, unable to think, and unable to respond. That is, apparently, they seem to stop thinking in these conditions.
References:

Wikipedia. Frontal lobe

WebMD. What Is Encephalopathy?

Roger F Butterworth. Chapter 38. Metabolic Encephalopathies. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition.

Wikipedia. [Delirium](https://en.wikipedia.org/wiki/Delirium