A depressed patient describes his thoughts as coming slowly and with effort, complaining of a mental fog that makes it hard to concentrate or make even small decisions, a pace that has gradually worsened over weeks. What is this?

This is inhibition (slowing) of thinking: a marked, gradual slowing in the pace at which thoughts arise, with difficulty concentrating and making decisions and a vague mental fog, exactly as described. It is not thought blocking, which is a sudden, complete stop in the train of thought rather than a general slowing of its pace. It is not dementia, since inhibition of thinking involves a reversible slowing tied to mood state rather than genuine memory and cognitive impairment. And it is not a formal thought disorder, which concerns the structure and organisation of thinking rather than its overall pace.